Do I need new tubes?

See if you can find a good audio shop close by with sophisticated tube tester, e.g., AVO or TV-7 and have them tested. Or swap all out from back ups, if you have them. Last resort, find a decent tube tester on ebay. I bought a B&K 606 for about $150. Basic good/bad on a scale, shorts and gas emission.  I own five tube amps and have over 250 vacuum tubes in collection so it was worth buying. 🤪 Just another expensive hobby. 

@rvpiano 

A few questions.  What kind of tubes are in your preamp?  How many tubes are in your preamp? How long have those tubes been in the preamp? And how old is the preamp?

Get some high quality vintage tubes. Install.  Fixed.

@curiousjim 

I have two Telefunken AU7 tubes which run the line stage of my Conrad Johnson preamp. I’m not sure how long they’ve been in the preamp, maybe a year and a half,

The preamp is a vintage model which was modded by CJ with Teflon caps.

That helps, but also raises the question, did you buy NOS or "used test strong" Tele’s? They may not be as strong as they could be. And honestly, the Telefunken 12AU7 is not one of their best efforts, IMO.

The 7316 is by far the best 12AU7, but they are quite expensive now. The RCA clear-tops are highly regarded and are not much more expensive than new-manufacture varieties. I happen think the Genelex Gold Lion small signal tubes are pretty nice, so that might be a way to perk up your CJ.