Only difference between the M(aster) and S(lave) amps is the volume wheel. M has it, S does not. Volume settings for both amps is generally done by remote control anyway. Unless you need to use the wheel, no need to pay extra for the M. Although it is a pretty sweet wheel !
For single-amping a pair of speakers, a 2150X amp and transport are all you need. This should give you good full range sound, with room correction. There is no need for a DAC or preamp, so keep that in considering when evaluating the price.
If you ever plan on adding subwoofers and/or multi-amping your main speakers with DSP crossovers, the 2150X is a poor choice. You'd be much better off getting an RCS 2.2X and 2150 amp(s).
The stock TacT amps sound pretty good, much better than I expected given their price. Just over a year ago, I replaced a Jeff Rowland 302/4 (ICEpower) with two S2150 and had no regrets.
But there were shortcomings compared to an excellent hybrid amp I later auditioned, the Blue Circle 204. Well over twice the price of the TacT, it has the same power output into 8 ohms and lacks the DAC & signal processing capabilities of the TacT. But the Blue Circle was more transparent and had cleaner high frequency reproduction. Also the TacT amp sounded somewhat thin across the midrange in comparison. The TacT amp did win on the woofers and subwoofers, however.
I know George found the TacT amps inferior to his DNA500 (a world class solid state amp) but I don't remember how he characterized the differences.
Generally, I believe the advantages of room correction will more than offset differences in absolute fidelity between the TacT and "better" conventional setups. Fixing the problems in my room were far more important to my listening enjoyment than some nuances of audiophilia. But for perfectionists like myself that can afford it, there are ways to realize a lot of untapped performance in the TacT gear:
1. Optimize digital signal transmission. This means using a very high quality source, good digital cables, and upgrading pulse transformers in the amp. If an RCS is used, its pulse transformers must also be upgraded, and a master clock should be installed.
2. Eliminate/minimize noise contamination. This should be addressed within the unit itself, by installing EMI/RF absorbing material on certain chips, wires, and circuit board traces; and within the system in general, by blocking noise into and out of each at the IEC.
3. Provide clean power. Use a power conditioner like Running Springs Audio or ISO-21 strips that filter noise on hot, neutral, and ground lines. Good power cords also help. I've had great system synergy with the Michael Wolff carbon source and gain cables. Annother experienced TacT user I know, who happens to be a reviewer for Stereo Times, is very fond of the more costly Shunyata Anaconda Vx which he uses with a Hydra 8. In the RCS, just upgrading the power supply (my first TacT mod) made a substantial improvement in the sound of the entire system.
4. Upgrade passive parts. The TacT amps aren't terribly expensive, and like nearly all high end equipment they are most definitely built to a budget. You can go crazy here -- caps, inductors, diodes, wire, Mosfets, binding posts, etc can all be improved upon. I believe this applies to nearly all high end equipment (excepting rare cost-no-object designs from the likes of DarTZeel, FM Acoustics, ASR, Boulder etc) and should not be interpreted as the TacT requiring extensive parts replacement to sound good. On the contrary, it took a while to develop mods for the TacT amps as they stock design worked quite well.
