Bass leaves after amp warms up?

I don't know as much as many posting here, but the amplifier/speaker match always comes up sooner or later when we know speakers aren't playing up to their potential especially in the bass. I've always seen in the forums that proper matching of the amp/speaker is crucial to proper performance. Sounds like a bad match between the Synchrony 1's and the Musical Fidelity amp. You may want to try a high current amplifier that "doubles down." (the power of the amplifier doubles as the impedance of the speaker is halved). By the way, Bryston and Musical Fidelity amps do not do this.

Interesting question I think Ralph may have hit on. I have lots of amps and they all exhibit the same behavior potentially. But I don't think it's limited to mosfets. I have both mosfet and bi-polar types and the same thing applies to both. This phenomenon is more prevalent when an amp is listened to "cold" and as it warms up, performance seems to fall off a bit. Not as noticeable with an amp left on 24/7. Mine are on 24/7 and sound better and better as they warm up. Why? Because quiescent current is higher when it's cold or cooler than when it's 'cookin'. So my solution is to set quiescent current to optimum when it's good and hot. The op has good ears. Not many people care about that even if they notice. It has to do with manufacturers' warranty issues. The cooler they run, the longer they last.

I am finding my Icepower Class D amps by far are less subject to dynamic compression at high volumes than any other SS amp I have used in recent years, including 360 w/ch Carver, 180 w/ch Tube Audio Design (I still own and use this and have a/b compared to Class D Bel CAnto), and 120 w/ch Musical FIdelity A3CR.

Only with the BC ref1000m Class D amps (500w/ch) does the volume continue to go up proportionally to raising the volume and the music continue to expand without compressing at some point prior to target lifelike volume levels.

The Class D amps seem to perform more like tube amps in their ability to go louder and louder without compression and fatigue, an attribute often attributed mainly to tube amps otherwise.

The 180w/ch TAD Hibachis (same ballpark power-wise as OPS MF I would sy) are lovely sounding amps even at fairly decent volume, but they do run out of gas sooner and at a point that some who like to listen loud and clear might find deficient.

To answer your 3rd question
If I turn the amp on and everything sounds good I listen as long as it lasts(about one hour).
I have not tried turning the amp on and waiting an hour to play music.
Also after the full sound leaves I tend to either reconnect speaker cables(which works if I am persistent) or come back later.
It's safe to say I never know if things will be the way I like them with full deep punchy bass or white washed and un emotional.
It's really perculular
Generally the amp plays better cold than warm but sometimes The sound dosen't change for a few hours,but inevitably does.

Foster_9, that's true, some amps are better than others but his amp plays fine for couple of hours and then looses bass. Switching source or adjusting speaker cable can bring back the bass performance instantly.