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

The Cary Audio SLP-98 new comes in right at your stated price goal. That’s with some upgrades tossed in. I’ve had mine for over a year now and it’s a keeper.

For under 5k used you can get a rogue RP-7 and some nice NOS tubes. I'll agree with bdp24 though when he says a preamp is unlikely to solve your problems. If there's not enough stuff in the room and it sounds too live maybe appeal to your wife by seeing what could be added to soften thr room. Like put a rug down if it's a wood or tile floor. Or get some window treatments if there's a lot of glass. These types of things, while not nearly ad exciting as buying a new piece of gear will make a marked difference in how the room sounds.

I’d get a preamp using the 6sn7 tube. Mod Wright, Cary, Supratek, Aric among others make tube pre’s using this tube. Many Pass owners like the Cary.

My LTA MicroZotl mates very well with my Pass labs X250.8, and you wont find a nicer company to deal with! I have rolled the tubes, with further improvement

Yes, get the MicroZOTL Preamp and don’t look back. You will not be disappointed.