Pro-Ject Tube Box S2 Review: A Tube Phono Preamp Worth the Step Up

Introduction

A phono preamp is the part of a vinyl system that many listeners upgrade last and hear first. The Pro-Ject Tube Box S2 is a vacuum tube phono stage priced at $500, which puts it at a point where the money starts to matter. That price has held steady rather than climbing over time, and open box or scratch and dent examples occasionally turn up from retailers such as Crutchfield.

If you are new to turntables, the reason you need a phono stage at all is that the signal coming off a record player has to be amplified twice. The first stage brings it up to line level, the same level as a CD player, streamer, or tape deck, so an integrated amplifier or receiver sees a consistent input. Most high end turntables do not include that first stage, which is why external phono preamps exist across an enormous price range, from around $20 up to several thousand dollars.

The Tube Box S2 uses an ECC83 / 12AX7A vacuum tube and is built in Slovakia. It has noticeably more physical heft than the budget tube phono stages that sit in the $50 to $70 range, which generally share one common chassis design with a variable gain control on the front panel.

Who this product is for

This is a phono stage for listeners who already own a turntable without a built-in preamp, or who have one that can be switched off. It is also the natural next step for anyone running a budget tube phono stage and wondering what a serious jump in price actually buys.

It is worth checking one thing before buying. Many entry level turntables ship with a phono preamp built in, and if that circuit cannot be bypassed, adding an external stage is not possible without changing the turntable itself. If your turntable has no preamp at all, an external stage is mandatory rather than optional.

Because the Tube Box S2 supports both moving magnet and moving coil cartridges, it also suits listeners who plan to move up the cartridge ladder later, as long as the cartridge loading works with the settings available here.

Main strengths

The front panel keeps things simple. There is a selectable gain control with six positions, and a subsonic filter switch on the left side. Everything else is handled underneath or at the rear.

Sound quality and noise floor

The presentation here is clean rather than syrupy. Tubes are often described as warm, but the character on offer is closer to clarity: a wider sound stage with more depth, width, and scale, more precise image placement, and a lower noise floor than a budget tube stage.

Measured against a typical $50 to $70 tube phono preamp, the difference is obvious and immediate. It is not five times better in proportion to the price gap, but it is comfortably two or three times better in the qualities that matter most. The character also lands close to the iFi Zen Phono, which now sells for around $200, with the addition of tubes and a larger sound stage. For listeners chasing the last few percent of what expensive phono stages deliver, this gets remarkably close for a fraction of those prices.

Flexible gain and cartridge matching

Six gain settings are available: 40, 43, 50, 53, 60, and 63 dB. The right choice depends on your cartridge, and cartridge specifications normally state the gain they expect, so matching them is a matter of reading the documentation rather than guessing.

Finer adjustment lives on the underside of the chassis, where a bank of DIP switches sets impedance and capacitance. It looks intimidating at first, but the logic is straightforward: there is a left channel and a right channel, and in most systems both are set identically. The switches are small, so a small tool and a careful hand are needed.

Moving magnet and moving coil cartridges are both supported. For moving coil use, the load is fixed at 47 kOhm, which is aimed at high output moving coil designs rather than low output ones.

Subsonic filter and connections

The subsonic filter is a genuinely useful feature. Very low bass energy from warped records or footfall can push a woofer in and out without producing anything you actually hear, and the filter blocks that energy before it reaches your speakers.

At the rear the layout is refreshingly plain: an RCA input from the turntable, a ground terminal, and an RCA output that runs to an integrated amplifier, receiver, or preamplifier. There is no power amplifier connection here, since a phono stage feeds a line level input.

Things to consider before buying

The honest sticking point is the price. At $500 this is a significant spend for a phono preamp, and the value sits almost entirely in the sound stage and the low noise floor rather than in features. A price closer to $400 would make the decision considerably easier. If those sonic qualities are exactly what you have been chasing, the money is easier to justify than the spec sheet suggests.

Setup also asks a little more of you than a plug and play box. Matching gain to your cartridge and setting the DIP switches correctly takes a few minutes and a look at the specifications, and the switches themselves reward patience.

Tube replacement and long term cost

The supplied tubes sound perfectly fine, so there is no urgent need to change them. Replacement tubes remain available because the ECC83 / 12AX7A is a common type, though they are not necessarily inexpensive, and prices for vacuum tubes in general have been unsettled. Treat tube cost as an occasional future expense rather than an immediate one.

Buying advice

If you want a tube phono stage and your turntable has no built-in preamp, the Tube Box S2 is a strong long term choice that should keep pace with cartridge upgrades for years. Check two things first: whether your existing turntable’s built-in preamp can be switched off, and whether the gain range matches your cartridge. Then look at open box and scratch and dent stock, which can soften the $500 entry price.

If tubes are not a priority for you, a solid state phono stage in the $200 range delivers a broadly similar character and keeps the spend lower. Buy the Tube Box S2 because you specifically want tubes in the signal path, not because you assume tubes are automatically better. Buyers who value a wide, quiet, well organized sound stage will feel the upgrade immediately.

Conclusion

The Pro-Ject Tube Box S2 is a well built, flexible, genuinely quiet phono preamp with a sound stage that makes the step up from budget gear easy to hear. It is not cheap, and $500 asks for a real commitment, but it delivers a large share of what far more expensive phono stages offer. For vinyl listeners who want tubes without changing the rest of their system, it is a purchase that is easy to live with.