Hi:
I have Mahlers and have used four different amps with them.
A Bryston 4B-ST worked fine and, provided the Bryston is surrounded by first-class associated components, especially interconnects and speaker cables, the sound can be surprisingly good. That would be a relatively inexpensive option.
I ran my Mahlers for two years with a VAC Renaissance Mark III 70/70, which is rated at 70 watts/channel. This would at first glance seem to be a mismatch, but, with all due respect to the above post, there are tube amps and then there are tube amps, and the VAC 70/70 is a beast. In a 2,500 cubic foot room, the amp made the Mahlers go really loud, cleanly, and with more than adequate bass control. An important consideration was that I was running Kimber Select all-copper and then Kimber Select all-silver speaker cables, both of which have unusually good bass control (Sumiko, the Vienna Acoustics' distributor, has demo'd the Mahlers at shows with Kimber Bi-Focal, a more modest cable). My dealer's impression of the sound was that the VAC 70/70 was great with the speakers and said that I definitely did not need additional power (his only suggestion was to get rid of the standard power cord on the amp for something better).
I bought VAC 140 Renaissance Mark III monoblocks (appr. 130 watts/channel) for my second system, and ran them with the Mahlers for a few months. I had a bit more head room as compared to the 70/70, but the improvement was subtle. For the record, all VAC amps were run with zero negative feedback with the Mahlers, and from the 8 ohm tap on the 70/70, and from the 4 ohm taps on the 140/140's.
I now use the VAC 140's with Salons and Rowland Model 6 monoblocks, with battery power supplies, for the Mahlers. The Model 6's are surprisingly close to the VAC 140's in all areas on the Mahlers. They are very refined and very powerful for their wattage rating (appr. 180 watts/channel), and are even more powerful if used on a 20 amp circuit straight into the wall without the battery power supplies (the stock power cords with the Model 6's are 20 amp cords).
As for controlling the bass on the Mahlers, the VAC and Bryston amps were equal (I know it's hard to believe that a 70 watt tube amp could control those woofers like a nearly 300 watt Bryston, but yah' get what you pay for). The Rowlands, however, do control the bass a bit better than the other amps.
Addressing some of the points raised in previous threads, the Mahlers usually have a big hump somewhere below 60 Hz. in most rooms that can make them sound boomy. The best way to address this is to use appropriate speaker cables with (i) a beefy solid-state amp like Bryston or (ii) an amp like the VAC's or Rowlands that have really high quality power supplies. If you are budget conscious, the Bryston 7B-ST's mentioned in the above post would probably be a really good way to go. On the other hand, Rowland Model 6's can now be had for less than $4,000 used (they were $10,600, new). To summarize, I think, overall, they sound better by a hair on the Mahlers than the VAC's (and a quantum better than the Bryston) - very refined, transparent and musical, and make no mistake, LOADS of power (incredible build quality and looks, too). Only if you are using the Mahlers in a gigantic room could I imagine you needing more power.
As for Krell, I have never heard Krells drive Mahlers -- I would imagine you could get good sound. I believe that Anthony Cordesman, reviewer for TAS, used a Krell FPB amp with the Mahlers in his review and reported excellent results.
Good luck
