Any Rogue Pharaoh or Musical Fidelity 5 or 6 si sound differences.

Agree with using NOS tubes with the Pharaoh. I use Nos Mullards, and they take the sound to a whole new level, better than the Cronus Magnum series. I'm sure it will vary from system to system, but in mine, bass is not weak sounding. I guess it's all about perception as far as bass is concerned.

I don’t have a Pharoah but a Hydra.  I am a big fan.  The Class D amps from Rogue are unremarkable with stock JJs.  You will need to roll in NOS Mullards for a warmer sound or NOS Ciftes for a more dimensional sound.  I also have a pair of NOS RCAs that made the amp unexciting.  The Cifte’s improves bass performance, the Mullards deliver sweetness.  

I am an MF dealer and they are awesome for the money.  Sound profile is similar.  Slightly forward and detailed.  The MF has better bass out of the box and the M6si is a pretty cool amp when you consider it is A fully balanced Dual mono design.  
The M5si is nice but less of a standout but is also quite a bit less expensive.  
One note, M6sis are out-of-stock at the distributor till October 02.  I have one on order for a customer.  

Having owned the Pharaoh but not the MF, I'd take a gamble on the 6Si. There is nothing remarkable about the former other than weak bass extension.