They are few and far between. I purchased a pair of fpb-750mcx blocks thru Audiogon a year ago (there were 2 sets available at that time - both in Canada) to replace a KSA-300S that wasn't up to the task of being able to properly jam the bass panels on my bi-amped Apogee Divas. Each Krell has a dedicated 30amp wire; as do each of my Audio Research 610T's (upgraded to run KT120s), which run the highs and mids. Many probably scratch their noggins over having this much power on hand, but I suspect those folks haven't heard the magic of Divas when properly powered. My KSA-300S ran really hot and would actually throw its breaker...not surprising for what I was asking it to do. The 750s do NOT run super hot like that at all, but they are warm. Of course the tubes on the AR's are giving off considerable heat anyway. Luckily I have a separate Hvac system for my listening room, so it's not an issue for me. Zero complaints about the big Krells.
Big Krells Have Vanished From The Used Market
Once upon a time several years ago, you could always find several of the big Krells for sale on Audiogon. I'm talking the 500-750 wpc big irons. Now since Krell no longer makes anything bigger than 500 wpc, anyone with the 600, 700, 900 wpc amps are holding on to them because there's nothing new by Krell to replace them with.