The KSA-250 has is esteemed by many and considered one of Krell's finest efforts. It has all the attributes you've described, and there will be no problem with the bass.
I own 3 of them. I've not heard the preamp in question. But from other posts would assume it sounds great. You might look for one their latter preamps.
I just love these older Krell amps, especially the ones without fans. Fair warning, these are older amps and will probably need some expensive maintenance soon, such as new caps and re-biasing.
The S series are very good amps [i owned a 100s] however when I replaced it with a FPB 300 I was not ready for the improvement. much better in all areas,
Only the early series Krell amps were truly class A. The orginal KSA series, KMA series for example; the S series was not class A.
I was one of their early dealers, had the KSA -50, KSA -100 and KMA 100s. Were excellent for their time but if it were me I would get a new 300i. Cheaper, newer, lighter and better sounding with plenty of power. Not likely to need the repairs that Unsound alluded to. If you thought they had a warm sound I suspect that was the influence of the pre amp. I had a different reaction to their early pre's, I thought they were dead sounding and would demo the amps with other preamps much of the time, the amps themselves were not what I would think of as warm.
I completely disagree with Kenscollick on the KSA-S vs the FPB sound. The KSA produced a warm, solid soundfield where the FPB had a bunch of smaller caps that would refill faster for quicker transients but sounded thinner & in no way approached the sweet, powerful sound & bass of the "S" series. BTW, I owned the KSA-200S & 300S as well as the FBB300 which the latter I sold rather quickly.
I have owned several of the Krell amps over the years. The best one I have owned was a KSA 80B and the second best was a KAV 2250. The 2250 had better bass than the KSA 80B but the midrange on the KSA 80B is magical. The other Krell amps that I have owned were A; FPB 600(It sucked), KSA 200S(It sucked), and a pair of FPB 750M Mono Blocks (Powerful sounded good but not great) I found the KAV 2250 stereo 2 channel sounded better than my big mono blocks when one of the monos went back to Krell for repair and the local Krell dealer gave me a KAV 2250 loaner. I now own Pass Labs X350.5 and it sounds better than any of the Krells I have ever owned.
I have a fully restored KSA-250. Krell charged me approximately $750 to do the restoration, mainly capacitor replacement. To my ears the KSA-250 sounds perfect. It will drive virtually any speaker to ridiculous levels and sound effortless. Compared to other power amps the sound is neutral albeit with extremely well controlled and deep bass. The only drawback I can think of (besides being ridiculously heavy) is, it will definitely heat up your listening room. This can be a problem with small rooms in the summer. This is an amp I will never part with, a keeper for me!
KSA 50/100- sweet and warm sounding Class A fan cooled see:
JA still has his 50!!!
KMA 160s- Pure class A- heatsink cooled- AH those fins! Warm, and huge sounding- very realistic timbre.
KSA 80- same as above but less power and a little less magic.
KRS 200- cut form the same cloth as the two above but Elephant sized in both power (200 real class A watts) and sound.
KSA 200- more neutral than the former three, some say more "white sounding" I say more true to the recording.
KMA 400s- same as KSA 200 times two, rare and wonderful, less bias current than KSA 200 if set to factory spec. Bigger stage, more detailed.
KSA 250- take every amp Krell made to date, put in a blender and you have the sound of this magnificent amp.
All S series Krells- sliding bias, just a little more mechanical sounding than the prior Krells. These have many fanboys.
FPBs- big improvement on the S series amps. Warm plus big, plus detail, lots of power at all specs. Many people think these are the best.
Cast series--- I don't have data or listening notes on these. Fellow reviewers love them. I don't buy into the "cast" technology- it's too limiting on equip matching.
I owned a KSA 250 for quite a few years. I did have to send it to Krell after a flame came out the top.
I think the repair was about $500 including freight shipping. The unit was returned installed on a pallet and a new box.
I eventually sold the Krell for a Pass Labs X350.5 Still about the same weight and it also puts out heat, not quite as much as the Krell.
The Pass Labs is a much better sounding unit.
I don't think the KSA 50 and 100 Mk 1s had fans. I don't remember them. The 100 weighed over 100 lb. if I remember correctly. Had VERY large heat sinks.
A friend of mine had the 100s driving his Thiel 3.7's. He then borrowed a 200s and preferred it because he immediately noticed more bass and overall a more balanced sound because of the extra power. He bought the 200s.
The original KSA 50 and 100 had fans in both their Mk I and II versions, as I recall--I owned a KSA 50 Mk II, one of the sweetest solid state amps around for a long time, and one I should have kept if only as a museum piece. Starting with the KSA 80 (which I also owned)/200 Krell started using heat sinks. Stan, are you thinking perhaps of the big KRS amplifiers that came out around that time? I far preferred the first three series of Krell amplifiers (through the KSA 250) to any of the sliding bias Krells that followed (especially the -s amplifiers) until you got to the FPBs. I'm sure my memory is overlooking the weaknesses of the early Krells, but they really could drive any load and I enjoyed my time with them, before the tubes started seducing me.