And B&K owners here?

The EX442 is 200 into 8, 360 into 4 with 75 amps peak to peak, while the ST140 was only 105 wpc and 28 amps I think.  The EX442 just sounded much beefier.  I had them at 2 different times so I couldn't A/B them.  When I sold the EX442, I immediately hooked up a pair of custom Paoli M70's (60 wpc - 8417 monoblocks) and immediately remarked that it was extremely difficult to tell the two amps apart.  The EX442 Sonata is that tubey sounding.

Which is better?  I think its all about system synergy and the speakers you are using.  Both are great amps & there are many who think that the lowest powered amp in a mnfr's lineup sounds best because there are less outputs in the amp.  Many think power corrupts (I'm talking hifi here, not politics :)

Thanks for info. I thought about the Musical Concepts mod, BUT, I'm still thinking new amp. The new Yamahas really look appealing to me. I can catch a refurbished SR2100 for about $2500 on Assesories4less. It's just waiting for one to pop up. The SR3000 really looks nice, but for $6000 + we are looking at McIntosh territory. I know I could get a vintage MAC for $2500 -$3000, but I think I'll go new

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.