And B&K owners here?

I have the ST 140 (105 w version) powering my ZU Omen Defs. Great low end and very smooth, non fatiguing mids and highs. More than enough power ( my Zus are 101 Db efficient). 

Although I love the amp, I will be upgrading this year. I’m thinking of a new Yamaha AS2100 or a used MAC. I want SS.

I thought about the Musical Concepts B&K upgrade, but figured I should go newer.

I heard a Yamaha as2100 a while back with the standard Omens and was taken back how deep and wide the music sounded. It was fantastic.

I’ll hang on to my B&K for a second system. I had a custom faceplate made so it would fit in cabinet.

Just curious how how others like their B&K amps and what they upgraded to?

I had an ST140 many years ago and also the EX442 Sonata, both were great amps and very tubelike.  I replaced them both for tube amps.  Scary how tubey those B&K's sound.

If replacing the ST140 for another SS amp, I'd look for Mosfets like a PS Audio Vs. getting an amp with bipolar outputs.  I think the Sonographe and cj solid state amps are very warm sounding as well.  VSP Labs were incredible amps back in the day too.  Perreaux, Tandberg, and Aragon made nice amps too but never heard them.   I did have an Acurus at one time, an 80 wpc one, was warm like the ST140 but probably not any better....Forte, Muse, and PSE are also other very musical amps as well.  

Just curious, how much sound characteristic differences between the st140 vs the ex442? Which did you prefer?
Loved my B&K ST-140! Bought it n 1985 and sold it here on AG in 2012. If I had it now I would consider talking to Musical Concepts regarding a refurbish and/or mod.  
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. 

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?
Thanks Lou. I was very proud of that system for the money I had in it. I still own the Sonograph TT. It was my only source as I did not jump into digital until the year 2000, never thought early digital sounded as good as vinyl at that time. Now mostly digital, computer based. Curious as to how it would compete against todays budget turntables. 

The Sound dynamics were more detailed, faster however not as deep in bass, more dynamic. A truly great speaker at its price. Now my Vandys were a very early model 2 (1977-1978).  I sold them and then they were returned to me several years later with non functional drivers. Cost to repair was too high so I gutted them and made bass traps with the enclosures. 
I had the Vandy 1B's a long time ago, I thought they were very pleasing sounding but not the most accurate.  I think I prefer the more accurate British 2 way monitor sound to the Vandersteens.

I'm sure your Sonographe TT would still compare very favorably to any of the budget TT's today.  I must admit, at 56, I'm still a bit stuck in the 80's when it comes to audio gear and I still believe most of the older stuff also compares very favorably with today's stuff.  More nostalgic and fun-ner to own too!
As my system now resides in a bedroom, I also prefer 2-way monitors. Currently a pair of Esoteric MG 10s.

I upgraded my B&K ST140/Pro10 to a Vincent integrated then went separates. Currently using a BEL 1001 MK5 SS amp and Jolida Fusion tube pre.  
I've temporarily downsized with a small Topping TP22 into a pair of Fostex back loaded horns from Madisound

with a small Atlantic Technology subwoofer while I'm waiting for 3 custom single ended tube amps (two 45 amps and one 3E29 amp) to be looked at by a technician.

I sold off my beloved Paisley Research AE500's as they were too large and hadn't used them in a while and my Onix OA20/2 developed a problem and I gave it to a friend to see if he can fix.  I'll then decide which tube amp I want to keep.  The Fostex's were acquired specifically as a budget speaker for the 45 amps.   
For a budget system in a small room the Fostex speakers look interesting. Especially when paired with a small sub. Any idea what the impedance and sensitivity parameters are on them? 
No, have no idea other than what is on the website.  If you dig a bit further, the Fostex speaker used has all its own specs on a different page.  I think they are at least 90-91 db with a very benign load.  I'm hoping they will work ok with the 45 amp I have.