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.

you could look at model 6's or M-10's, or M-12's, etc. the M-8 usually sells for $7500 or more if it's in really good condition. as nice as it is you could be overlooking some outstanding amps by Rowland that do come up for sale.
an M-10 makes 150W for example, the same as the M-5. all Rowland amps are overbuilt so 4 ohm speakers should NOT be a deal breaker.
Thanks, FF. I didn't know a used Model 8 commanded that high a price. That's a bit out of reach for me.

I was reading up on the Model 10 a bit. Some folks stated they thought it was a bit lean sounding compared to the 2/6/8/9 series. On person even said he had a power supply problem with his that couldn't be fixed by the JRDG's importer. All people report the 6's were superlative. Anyone know what a pair of these go for used? And, I know, good luck finding some.
I know this is off topic, but I saw a Bat VK-500 w/Bat-Pak for sale here. I've never heard any BAT SS gear, have read some reviews, seems like it may be similar to older Rowland gear but with better bass definition. Any thoughts?
Billspecfoc, maybe I can give you a little insight. I own a model 8 with choke and I have heard the model 5 on several occasions. The difference between the two isn't minor. The 8 has tremendous bass definition, while the 5 is in no way weak in this area, it isn't able to plumb the depths like the 8. BUT, that's not the biggest difference between the two. IMHO, the biggest gain is the 8's ability to resolve a LOT more detail than the 5. As for whether the 8 could drive your speakers, I'm in agreement with French fries's post. IF you can source an 8, I think it would be a great addition to your system, I know it was to mine.
Good luck with your search.
Memories are coming back ... In the 90's I owned Model 1 (my Start with High End...), Model 3 and later Model 8. My way at that time was going to high sensitive Speaker Designs, so I went to low powered amps later. In all those years when I listened to a lot of Systems, I never heard that "Magic", that kind of "realness" from any Transistor amp again (Until this year with a Model 2 + Batteries in combination with a high sensitive Speaker). That was (and is) one of the very few timeless Designs which was simply done right. Listening to music with Model 8 is really outstanding. These Designs - 2 [6], 8 [9] - made JR (with Consummate Preamp) famous, even among very picky, serious listeners I met...
Thanks everyone. Keep the comments coming. I soak this stuff up like a sponge.