Help with power amp

Mosfets are far from fragile in my experience. Two of my amps are Mosfet designs with NO protection in them whatsoever other than fuses on the mains. I have pounded on these things with very low impedance loads time and time again and they have never even flinched. My brother also has several Mosfet based amps that we have removed all of the protection from. He used to drive his amps into clipping quite frequently before he went into a multiple amp arrangement. No problems for him either.

As to the different amps that you have listed, i'm simply going by past experience with the "house sound" of MF and Monarchy. The Musical Fidelity gear is typically quick and lean with good detail. While it does lack "slam", "impact" and "warmth" compared to many other amplifier lines, they are quite smooth and easy to listen to in comparison to most other SS amps.

The Monarchy's are quite different from the MF's in my experience. Much warmer and more relaxed sounding overall. While some might think of this as being "tube-like", i really don't think that they sound like tubes. Whereas the Musical Fidelity amp might come across as "perky", the Monarchy's typically tend to sound "romantic". Depending on the overall tonal balance of the system as a whole, one amp would be more suited than the other. I don't think that they would be normal candidates in terms of being "swappable" within the same system without MAJOR differences.

Can't speak for the ARC stuff. Not familiar enough. Hope this helps... Sean
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Sean's comments on the Monarchy and MF amps seem quite adequate. My personal preference is for the warm smooth character of the Monarchy SE 100s.

I believe the ARC D115 is an older, somewhat dated, tube design, which sounds nice, but exhibits a relatively high amount of noise in the form of tube hiss (certainly when compared to either of the other amps you've mentioned). Using the D115 with high efficiency speakers may disqualify it because of this intrinsic high noise.