I am one of the few here on AH that have owned B&K gear, so I thought I'd chime in. I had a B&K AVR 507 that I bought used in 2009, and replaced about 1½ years ago. My short answer to your question about the 507 is don't buy it. See below for reasons why.
In general, I agree with most of what others have already said.
My current system is needing repair to my Preamp ( B&K 4090) which I have been quoted at $200. I'm using a B&K ST 140 (70wpc) to drive 2 Medowlark Heron speakers. I have been very happy with this set up in sound quality and volume.
Nice speakers. I remember hearing them in an audio shop many years ago. You haven't asked, but you should certainly keep the speakers and the B&K ST 140 amp.
I can either get my preamp repaired, but no warranty, or purchase a B&K AVR 507 (7X150w into 8ohms) off local Facebook for $100( no warranty). The B&K does have the ability to convey all information into a 2 speaker stereo mode according to the manual. The AVR 507 doesn't have HDMI outputs ,so I wouldn't use it as a surround with my TV, but just to replace my amp and preamp.
I assume you are aware that B&K went out of business a number of years ago (2010?), and their customer support no longer exists. This becomes an important reason why not to buy a used AVR 507.
When I bought my AVR 507 used, it was shortly after B&K came out with a new model with HDMI inputs & outputs. Nervous audiophiles, eager to have the latest toy, were dumping their pre-HDMI units, and I picked up the 507. It was a very good system, and I liked it, with several exceptions. I used in a 5-channel HT/music system, and over the years I regretted not having HDMI. It made hooking up TVs, newer disc players such as a Blu Ray disc player, and other digital streaming AV devices, much more complex. You don't seem to have that particular problem. That AVR did allow for adding external amplifiers, as you had asked. But that is also available with other AVRs.
I would have kept the AVR 507 but during 2020 it's onboard software started gradually misbehaving. At first, I could ignore it, but it eventually got much worse. I could no longer turn it on or off. Other software-based control problems could pop up and vanish with alarming frequency. Apparently, the hardware of the AVR was working fine, but without the control software, it became useless. I replaced it with a Denon AVR-X4500H.
For what it's worth, the B&K AVR 507 was very heavy and got very hot as it ran. I don't know for certain if the heat contributed to it's demise.
Because B&K had closed, I could not send it to the factory for reinstalling the software. If you buy anything of the same vintage of the AVR 507 you should be aware of this severe problem. This lack of factory support also applies to the remote control that B&K supplied with this AVR. For both of these reasons, I recommend that you do not buy it. If repairing your old B&K AVP 4090 can really be done for $200, that seems a useful option.
My question is, assuming good working order, would the B&K AVR 507,being higher wpc, equal or perhaps exceed the sound quality of my amp and preamp combo? Spending a lot more money is not possible. You wouldn't believe how little money I have in the system now.
There are other 2-channel audio pre-amps or older (more reliable) AVRs that can be combined with external amps that will get the job done for you. I wouldn't worry about any loss of sound quality, as most of that comes from your speakers, not the audio pre-amp.
You mentioned you use an older CD player. Does it hook up to the pre-amp by analog (the old standard stereo pair of RCA interconnects), or does it hook up digitally with a single optical or coaxial cable? Is it capable of both methods? Does your old AVP 4090 have an on-board digital-to-analog processor that would accept a digital feed, or did it require an analog feed from the CD player? This might be of importance when looking for a replacement for the AVP 4090.
