Rowland Model 5, upgrade to Model 8/8ti

I'm currently running an old Rowland Model 5 amp, the fully differential model with SE and balanced inputs along with a Capri preamp. Cables are Kubala-Sosna Fascination IC's, Reality SC's (want to get KS SC's). CD/SACD player is a VSE Level 6 Sony 5400 with the Lite clock. Speakers are some heavily modded Reimer Tetons in which the only stock parts are the ribbon tweeters and the cabinets; they sound MUCH better than stock. They are 4 ohm speakers and quite current demanding.

My question is how big of an upgrade will it be going from the Model 5 to the Model 8/8ti? I really like the mids and highs that my current setup have and the amp doesn't seem to run out of steam. I knew when I bought it that it didn't have the greatest low frequency definition. This is readily apparent. If I went to one of the Model 8 iterations, I don't want to give up the mids and highs that I like now. I expect there would be better bass definition. Just how much?

Then there is the other problem of finding one. They don't seem to come up for sale very often. But I'm patient.

Bill