i'm looking to change the stock 12ax7 in my art tps ii and was wondering if anyone had any experience replacing the tube in said unit.

i'm looking for a deep/dark bite. anything worth trying?

12AX7 tubes are all very similar ... or at least, they should be. All tubes of a given designation should have the same performance characteristics (within tolerances).

To really change the sound of a tube circuit, it's the topology and supporting components that cause most of the sonic differences rather than the tube itself.

Otherwise see response from ambiguous signal

interesting. @ Rob Coates was quoted in a similar thread saying that a different tube made an audible, significant difference in the tone of the unit.

Of course there are pin compatable valves with totally different gain characteristics ie:
12AX7 - gain =100%
5751 - ~57% (of a 12AX7)
12AT7 - ~60%
---anything less will start to sound very different - perhaps ropy! Or characterful? ---
12AY7 - ~ 45%
12AV7 - ~ 41%
12AU7 - ~ 19%

The TPS II is mostly a transistor preamp. The tube in there is only one part of the device and isn't doing the real work of loading the mic or even doing much of the actual boosting of gain.

If you want deep and dark that's as much about the source and how you mic it but if you want gear that helps that along a bit I'd say that tends to be preamps with good transformers, dynamic mics, ribbon mics, and condenser mics with transformer output.

In the land of budget gear I think you would do better getting an ART MPA. Somehow that preamp although built mostly like your TPS II manages to sound a bit like they have some transformers in there even though they don't especially using the right mics and loading them heavily. The MPA is going to give you more "warmth" than putting expensive NOS tubes in your TPS for a similar price.

Dark with bite or warm and clear is the big ask in the land of recording. Generally gearwise you are talking about the more expensive preamps coupled with nice compression dialed in properly (probably with more transformers) along with a wonderful source in a room that isn't going to muck that all up.

I'd skip chasing the paper tiger
thinking you can make your TPS into something wonderful by simply locating the right magic glass bottle and instead consider what mics you are using, how they are loaded, and maybe getting a solid quality micpre that has some real character.

This is very true and a good option to look into.

Changing tubes on a ART TPS should be audible but won't produce significant change. It is not running on high voltage (45 volts if I remember correctly). It also uses only one tube (half of a 12AX7 for each channel). The Pro MPAII is an entirely different design and uses full voltage (well 150 volts anyway--not starved plate) on the tubes. I've found it much more sensitive to tube changes. I have JAN Phillips 5751s in mine and it did produce a quite audible improvement.
It's just a much better preamp overall than the TPSII. However, there's a guy in Watertown MA at Audio Proz that does a mod to the TPS that looks interesting. I believe he does something to add additional second order harmonics.

As mentioned, as a "starved plate" design there is debate about whether the tube is doing much at all - it may simply have been included in the circuit to give the consumer the impression that he/she is buying tube gear.

Rob mentions that he put 5751s in his MPAII and it produced an audible improvement so try 5751s as those tubes are a different animal (the design is about current rather than voltage) than the other gain variants in the 12AX7 "family". Although you can slide a 5751 in and it will work fine, it works in a different way from the rest of the 12AX7 family and if you are hoping to notice a difference (apart from a reduction in gain) the 5751 is the only tube in the 12AX7 family that could achieve that.

aka the 5751 is not just another lower gain version of a 12AX7 - it works differently - and could allow a different tone.