Replacement for KSA-50

I think your better off using the eq. The eq starts boosting at 70 Hz, not an area where most sub-woofers do their best work. If you want to use a sub, I suggest using the 40 Hz setting on the eq. I don't think the eq really causes that much compromise, and the benefits far outweigh any disadvantages (which might have more to do with preconceived prejudice?). You can insert the eq into a tape loop of your pre, if you'd like to switch it out when listening to music without any sub 70 Hz sounds, but then you'll be introducing more things into the signal path.

Yes, the Krell is great amp, but... those Theils wouldn't mind some more power, and FWIW, I never much cared for fans on amps.

I'd keep an eye on e-bay. They show up from time to time there. After securing one, you might want to send it to Thiel for inspection. A bit of needed maintenance on the Theil eq's is not unheard of. Thiel tends to be very generous about doing such things. Using an off the shelf eq might work, but then you'll have many more switches in the signal path, most of which have nothing to do with your intended purpose. I think Thiel was originally suggesting that one could use some of the after market eq's such as the JP Labs Golden Flutes that were being marketed during the time that Thiel was making the 3.5's. Those can be quite difficult to find now. Before Thiel ran out of replacements eq's, I believe they were charging $600 for a replacement. Comparable quality in a cheap multi use off the rack eq would seem like a rather dubious effort in my opinion, especially considering your previously posted concerns.