@coop_301 , Please indulge me as I offer some thoughts. IMHO, the CS 6's play with greater ease and with greater dynamic range than the CS 3.6's, at a very premium price difference (at least when new). If you have a rather large room or intend to play your music very loud the CS 6's might be a better choice, otherwise the better value could be the CS 3.6's. A contributor that I've long come to respect here on Audiogon @pops has extensive experience with both, hopefully he'll add his thoughts.
As for the Krell KAV 1500, it's really a bridgeable 5 channel amp. The Krell KAV series was introduced to cater to the then growing Home Theatre market. They are biased more into Class AB, don't have the low impedance prowess of the built for audio biased into Class A and capable of doubling down with impedances of some of their more audio centric models. The KAV 1500's 5 channels can be bridged, but that's not something that I would consider with these Thiel models and their rather low impedance loads. Not all Krells fit the stereotypical (ha!) model that comes to mind when one hears the name Krell. I'm not at all surprised that you preferred the CS 6's driven by the ML 23.5. I say this despite more often than not preferring the house sound of many Krells to Mark Levinson's.
When I was much younger, I believed that the most important component were the speakers and therefore most of one's budget should be spent there. I still believe that speakers are the most important component (after budget and room) but no longer necessarily believe that that should dictate budget. The used market has changed that. Market value and intrinsic value do not always run parallel. Speakers and amps are more dependent on compatibility than most components and this is especially true for these and many other Thiel models. Unless one intends to upgrade in the very near future, and caution must be used in that case, the combinations results can be more important than the individual prowess.
As always, the above is IMHO, YMMV!