USED pre-amp + amplifier combo for B&W 802 Matrix series 3 around or under 2000?

Check out some Bryston equipment.

A Toppping pre90 ($700). Supper neutral and hardly works with many amps (too much GAIN). Normally would not recommend this preamp but you need it under $2k and paired with a warm amp such as a used Paraound A21 ($1500) (with adjustable GAIN) will work. I once had the pre90 + Parasound A21+ and it was good with Thiel CS3.7 speakers.

This week I got the new Class A Schitt Mjolnir preamp. It is considered a headphone amp but it is also killer as a 2-channel amp. $1200

https://www.schiit.com/products/mjolnir-3

I compared it to the Holo Serene and Benchmark LA4 preamp in 2-channel. The other 2 are not going anywhere but the Mjolnir is a superstar for the price. It is now my headphone preamp connected to the Schitt Aegir 'Class A' 20-watt amp ($700).  This combo replaced a $7000 dedicated headphones amp. The Schitt combo is about 90-95% as good as the $7k amp.

This preamp will be amazing with your B&W. Schitt has the more powerful Tyr mono amp, that I have not heard, but they are more powerful than the Aegir. The Aegir can be run as mono to make 80-watts, but that may come with standard issues with using a stereo amp in mono. Dedicated monos are better.

I would consider the Mjolnir with the Parasound A21 for the B&W. The Mjolnir is a keeper and you can upgrade the A21 in the future.

A Peachtree GAN400 (I sold one for $1100 recently) would work nicely with that Schitt Mjolnir. I should add this this new Schiit preamp is much better than the Freya+ that I had before.

@OP. I suspect it will be a bit of a struggle at that price point. You need a power amplifier section with lots of current - and the capacity to deliver it into low impedance. Even secondhand, the budget to do that with good sound quality is a challenge. Bryston and Parasound are good recommendations. New, though I haven't hear the specific model , the Schiit Tyr looks like it will work - though it doesnt leave much in the budget for a pre.

You are being very optimistic.