Don't know which B&K reference, but it seems that your speakers need more power beyond vk-250SE capabilities.
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It's not the best model of VK and more than certainly it can't do 300/4Ohms(sorry only on 'holidays':-)). It does better job with lighter speakers than you have. So I suspect mismatch of components.
An upgrade to your B&K might be Wyred4Sound SX1000 monos
I was lucky to get Crown 3600 powerhouse(1200W/4Ohms) for low hundreds from thrift store and this piece pleasantly surprised me with stunning and solid performance/Aerial 10t! Fan is literally unaudible, undisputed crispy detail on any volume level. If you find it for cheap I'd recommend you to play with it...
I assume this is a used amp and therefore already broken in. I leave my McCormack amp on 24/7 (partly for sound quality but also because I don't like turning electronics on/off frequently), but on those occasions when I turn it off and it gets cold it seems to take a couple hours playing music before it comes back to life. Before that it sounds slow and mushy -- kinda like you describe, and although all amps are different I'd be surprised if the sound of the amp changes substantially after that warmup. Anyway, just thought I'd relay my experience.
I give my solid state CD player about an hour of "warm up" before I listen critically. Put on a disc, hit "repeat" and come back after an hour or so for serious listening. I turn on my tube amps and preamp for at least 60-90 minutes before I bias and start listening. Overall, everything needs to be running for at least an hour or two before the equipment "settles in". Amazingly, even the quality of my electricity will vary and effect the sound. I bet a lot of us here notice for some inexplicable reason, some days of listening are better than others. I attribute it to the oddities of supplied electricty. However, I noticed I have consistently more satisfying listening sessions since I put in a PS Audio Power Regenerator. Long and short, warm up the gear before you get too critical.
I think that Marakanetz may be correct.
I went from a Spectron Musician III Mk 2 to a Pass XA 30.5 and had the same thing happen to me, at quiet, simple passages it was great, but at loud or more complex passages everything went mushy.
I was told that it was the lack of power with the Pass causing the problem. I returned to the Spectron and everything was fine again.
There are two issues here: 1) whether you would eliminate the "mushy" sound by increasing the amount of warm-up of your solid-state gear, and 2) whether your BAT amp has enough power to properly drive your Dynaudios.
Starting with the second question first, wattage ratings for equipment are usually misleading and often completely irrelevant. What matters most, in the case of a solid-state amp, is the amp's peak power capabilities, which is determined by the size of its power supplies, and in the case of tube amps, the size of its power supplies and quality of its output transformers. There generally aren't surprises - the really expensive gear is where you find amps rated at 100 watt/channel that are subjectively more powerful than typical 400 watt/channel amps. BAT makes pretty fine gear and I suspect that you are not experiencing a power gap - Dynaudios do like power, yes, and are only of average efficiency, but they also have first-order crossovers and don't present a vicious load. And all things being equal, there is very little difference between a 395 watt/channel amp and one that's 300/watts/channel - if you double wattage, you only get 3 extra decibels of volume.
As for "mushiness", standby functions on solid-state amps tend to keep the input circuitry powered up, but not the output circuitry - unfortunately, it is the output transistors that raise the temperature of the amp, so if you toggle in and out of standby, it's the equivalent of turning the amp on and off, with the resultant harmful thermal cycles (heating up and cooling down). Over time, numerous thermal cycles will make internal components more likely to fail. The other problem, of course, is that equipment which has not yet come up to temperature doesn't sound very good, especially in high-resolution systems, and reaching thermal stability can unfortunately take anywhere from four hours to a week, depending upon the piece of gear in question. These are just general observations, however: I do not know how the standby function works on the BAT amp; there are some solid-state designs, such as Class-A biased amps, that are better turned on and off because they run components near their thermal maximum, which increases the likelihood of failure; 24/7 operation will decrease capacitor life (although caps are relatively cheap and easy to replace, and far less worrisome than losing an output transistor). I have contributed to numerous threads addressing whether or not to leave equipment powered up 24/7, and I suggest you look at my "Threads Answered", at "Amps/Preamps", if you want a lot more detail. The short summary of my views is, in the interests of sound quality and in preventing premature component or tube failure, as applicable, I advise leaving most solid-state gear, and most tube gear that uses only small-signal tubes, powered up 24/7. Tube amps have to be turned on and off because output tubes pass a lot of current and will fail relatively quickly if left on 24/7.
The suggestion about reaching out to Glenn Poors is a good one.
"It's not the best model of VK and more than certainly it can't do 300/4Ohms"
Says some random anonymous dude on the internet. Unless you can back this up I see no reason not to trust BAT's specs, especially given the massive power supply of the 250SE.
I owned a 250SE for about 8 months. The description is not out of line with the character of the amp as I heard it, but I was never willing to leave it on 24/7.
I would suggest the OP audition some different amps.
leaving the amp on or off 24/7 is absolutely irrelevant to this issue.
you can trust BAT specs but only for very short duration and for quiet music such as small jazz band, chamber(where OP describes sweetness of sound). in all other cases you need much more continuous power with OP speakers.
mentioning 'not the best model' implies that there are better ones for the same price range and value. BAT is superb gear but there are certainly a number of better ones for the money spent than VK250SE.
I only meant to qualify my subjective impression of the VK-250SE by saying that I never left mine on 24/7.
Again, you've offered no justification for your claim that the 250SE can't adequately drive these speakers.
I've no doubt that the OP is hearing what he is hearing, and like him I didn't particularly care for the overall sound of this amp. But I don't think that any technical shortcoming of the amp can be blamed.
I have yet to see a solid state device of any kind that did not benefit from at least 24 hours of warmup.
Certainly in this case its worth a try- if it makes no difference then don't leave it on all the time :) But I think you will find that it does make a difference.
Now- will that fix the mushiness at high volume? Hard to say. If it does not, its probably not because of insufficient power; if a speaker needs more than this amp makes then the speaker is simply impractical for the real world. But if there is not enough filter capacitance in the power supply (or if the filter caps in it are having some issues), mushiness at high volume will be the result, regardless of the power the amp makes or how long it is left on.
Tubes need only 5...10min to get all DC voltages steady while transistors need at least 25min to reach operating specified parameters.
In fact tubes are not sensitive to temperature at all while transistors are. That's why they need radiators so their temperature does not go above certain point otherwise the power will produce heat instead of current.