And B&K owners here?

Boy, give my $2500-3000, I'd put together a system that just cooks McIntosh and Yamaha

I have a B&K ST2140, a later more powerful version of the ST140, which was paired with Mirage FRX-7s and a B&K PRO-10MC. A lovely amp, I "upgraded" to an Aragon 8008BB, exploring "neutral, clean" sound with Vandersteen 2CEs, still with the PRO-10MC on pre duties. 8008BB aids with the discipline of the bass for sure. Not warm, accurate. Better detail, soundstage, so I guess I've moved away from tubey. Currently considering Schiit Freya as pre upgrade. So much to learn, trying to internalize importance of room interaction with system and that more $ is not necessarily better.  I wouldn't do the B&K soup-up route. Nothing against that company, though, know nothing about them. I had a 1967 Scott Stereomaster partially restored. While it's more reliable now, not sure it sounds much better. I know that's not a fair comparison, but the experience has pushed me to go under the hood myself, building confidence and learning about the wonders of audio. I repair job that I paid a tech $237 for cost me about $20 doing it myself on another unit. I know they are selling their amp soup-up expertise, but it just seems to be swapping out higher quality parts. Better to buy a newer unit with a great reputation with the parts the designer intended to have.  Then again. I don't have experience with modded units, so I don't wish to disparage. 

So why are you upgrading it? Looking for more power? If the Yamaha or Mac are of the same size, I doubt you will be moving up. Just a lateral move. If you think the Yamaha is going to sound sweeter or better, you may be disappointed. Those early B&K models had a great smooth sound to them. But an upgrade in power will give you more to work with, drive bigger speakers and fill larger rooms, but still may not actually sound better. Glad to hear you will be keeping it, so you can see how much improvement you get, or don't get. 

Way back in 1986 a B&K ST140 system was driving a pair of Vandersteen 2C's with a Sota 'table, at the local salon. I was completely blown away and moved by its performance, especially at its price point. I think I could have bought it all for $1200. But I had just sunk my money into an Adcom 555 and system before I got there. Oh I was still happy with what I had, but I was very impressed to say the least. Brings back memories. 

Back in 1985 I put together a system consisting of Sonographe SG3 TT w/ Sumiko MMT arm, Tasliman cartridge; B&K ST-140 and Pro10 pre; Vandersteen 2s (1978 models). Enjoyed it for about 8 years then replaced the Vandys with Sound Dynamics STi  300s. Kept the B&K gear 25 years. Felt the 140 to be the best of that system.

Nice.  How did you like the Sonographe TT?  I had a Rega Planar 3 for many years but always liked the looks of the Sonographe.  Also, how did you like the Sound Dynamics compared to the Vandys?