Overall, I agree with jcoat007. As a rule, if all else is equal or close, improvements should start at the source. I had my source fixed (Modwright 999ES Platinum) before I went the SWL9.0SE route, and found that, although both were huge improvements, the source was the right place to start. My dedicated analog pre at the time was no slouch; that being said, the new SWL is much much better (read my reviews, and others). However, if your Krell HTS is anything like my Lexicon pre/pro, the analog 2 channel section is below average (which is why I didn't use the Lex). I would be surprised if the HTS's analog section is as unmusical as the Lex. However, if that is the case, and I'm purely speculating here, then start with the pre. It will make all your 2 channel sources sound better, especially if you tube roll and find some warmer options in the meantime.
Edit: Just quickly scanned 2 reviews of the HTS and they both said the analog stages are some it its best features; so I believe it's a no-brainer to go the Denon mod route first. The SWL will be a nice next step.
Ted