Your question lacks context, and is therefore unaddressable. Could you please list your complete system including cables, etc. It very well may be a matter of properly configuring your system as it is right now, and not simply a matter of switching amplifiers.
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actually, I'm running 2 MC275's in mono for each B&W 805D's.Well that's a little more information, but not as much as I was hoping for. If you have your amps bridged, you have more than enough power, and more than enough damping, to get loud, tight bass out of your 805's.
Are your amps located right next to each speaker, using short speaker cables? What brand/model cables are they.
one foot long, each side short as possibleOrdinarily, a short(er) cable (anything under 5 feet is sufficient) will help preserve the limited damping capability of a tube amp. However with (especially) a braided cable, like the Kimber, a very short length may work against you because of the low frequency oscillations (some people call them 'reflections') that can form in a cable that has high inductance and capacitance like the braided Kimber. (This is common in undersized power cords also.) I would suggest you try a cable of 4-5 feet in length and preferably solid core (or very few strands.) If you want to do an inexpensive experiment, go to Home Depot or Ace etc., and get four 5 foot lengths of insulated copper house wiring, 10 AWG, and see if that doesn't improve your bass performance.
The (current model) MC275 develops 90w/ch and should have no trouble driving an 802 with its 90dB sensitivity (unless you have a large room and/or listen loud -- in which case you might want two MC275's in mono ;--) But it's important to note, that B&W specifies using very low resistance speaker cable with their speakers (probably for compatibility with their crossover insertion impedance), and that requirement translates into "fat copper conductor" which does not describe a braided cable (nothing against Kimber) but I know many people who love the sound of tubes with their B&W's, and they all use cables with lots of metal and very low resistance/foot specifications. Try it, it'll cost you maybe $10 ;--)
Why don't you just try some solid copper from HD as I suggested. I'm not recommending it as a permanent solution; but it will tell you right away if that (speaker cables) is the problem. Another thing you want to double check is your amplifier taps -- I'm assuming you're aware that when you bridge the 275's into mono, the impedance designation printed on the amp for each speaker tap is no longer valid and you have to go up a tap (i.e., an 8 ohm speaker should be hooked up to the 16 ohm tap.) Check the owners manual.
As for cables other than the Kimber, there's a pair of one of the all-time great workhorse speaker cables (Straightwire Maestro) available used (always buy speaker cable used ;--) on the Gon right now, for no money, in just the length you need: http://www.audiogon.com/cgi-bin/cls.pl?cablspkr&1273259411&/Straight-Wire-Maestro-SpeakerThere are also a couple pairs of Virtuosos, a later design, for a little more money:
I used both models for years (the Maestros for 10 years) and they are one of those sleeper products that are very hard to beat (especially for the used price!)
If you want to spend some bucks, a pair of mid-priced Virtual Dynamics. Most VD speaker cables come as two separate cables per speaker (a plus run and a minus run) with solid conductors. This completely eliminates capacitance and inductance. If you want to spend a lot of bucks, Purist Venustas or Provectus (the Provectus is solid core two runs like the VD) but you're probably talking $1200+, even used.
The MC501's are terrific amps. . The first thing I noticed compared to my MC252 was more bass, more 3D imaging. My MC252 sounded good but the imgaing was left, right and center. With my MC501's I sometimes hear imaging above my head and a sensation of layers ie a drummer playing behind a singer or guitar player. I am using them with SF Elipsa speakers and have not desire to change them. IF you buy the MC501's you will probably never want to replace them. I am very happy with mine.
I have two MC275's mk.V's running mono to power a set of Harbeth SHL5's and I love the sound. A truly epic experience to say the least! That being said, I am considering a switch to the MC501's for two main reasons, life style and a tighter control of larger speakers. The fact that I won't have to re-tube down the road and the fact that I can leave them on without worrying about degrading the life of the tubes is a definite plus on the S/S side. Also, with inefficient speakers like the SHL5's, 350 more watts of power would roughly equal 4db more dynamic headroom, which should help a great deal in controlling the drivers.
I have heard the MC501's and thought them to be fantastic, and think that it would be more of a switch of attributes rather than an overall loss. Nothing will sound like a tube amp other than another tube amp, but that being said, It is hard to match the sonic benefits of S/S amps with a tube amp. The Layering and dimensionality of the MC275 makes it one of the best amps around period, but the control and accuracy of the MC501's make them also one of the best amps around period. At the end of the day, it will up to a number of other factors that will help you finalize your decision, but I don't think you or I for that matter could go wrong with either one.