Looking for a new (preowned OK) line stage under $5K: suggestions?

Assuming your speakers are up against the view… glass. Tube diffusers behind your speakers… no higher than your speakers.. Wall behind add a decorative thick woven carpet. Diffuser panels at side reflection points. I don’t know what your room looks like adapt ideas.

There could be a key place you absolutely need a panel. Then make it self standing and slide it out of the way when not listening. The real key is to figure out what treatments are required… then figure out how to make them unobtrusive or movable. Test with a ladder and a hung set of blankets or some other proxy to figure out what it takes. Then think through how to make it happen either looking good or movable.

You can get these with any art you want covering the panels and they are only 2’ X 2’ and said to be very effective

How come no one talks about DSP as a way to improve acoustics?

Without hesitation, Audible Illusions L3A!

I paid $2000 for a mint condition Audible Illusions M3B, which was one of the best purchases I've made in audio. L3B (similar to M3B but with a remote and without phono) is also worthwhile to consider.