I need advise on amp upgrade.

I am interested in upgrading my Bryston 4BST amp.My system includes, Audible Illusions M3A preamp, Bryston 4 BST amp, Vienna Acoustic Mahlers speakers, Musical Fidelity A324 DAC, Sony 777ES SACD player, PS Audio P-300 with multiwave, PS Audio Ultimate Outlet high current version, Harmonic Tech Pro-Silway ICs & Harmonic P9 Plus speaker cables. My budget is $4000 for a used amp. My choices are Pass Lab X-250, GamuT D-200 and the Plinius SA-250 MKIV. Thanks in advance.
Pass x-250
I would vote for either the pass x-250 or cj mf2500a.
I would actually replace your preamp before replacing your amp. There are MUCH better preamps out there for similar dough than your M3A. In fact, I purchased a cj PFR which, in my opinion, blew away the M3A in tonal accuracy, rhythm and pace, and most of all, musicality. Keep your Bryston; get a new preamp instead.

However, if you are convinced about replacing your amp, the GamuT wins in my book. The Plinius is second. Both these are very musical amps and neutral to the source.

Good luck.
Best bang for the buck, in my not so humble opinion, is a used Krell FPB300.

Hello, If the speakers are efficient,the Gamut is good.However,if not you will tax this amp.Between the X250 and SA-250,the X250 will sound quicker in the top-end with the Plinius adding a little sweeter treble.I prefer PURE CLASS A operation,but the X250 has a fair amount of this to offer,then switches to AB for high power situations.Good luck! Tom