Preamplifier for Threshold 400A

Not knowing a budget Pass XP10 if you want to go SS. Tubes ModWright LS36.5 and many ARC preamps will work and sound nice on a Threshold. I'd suggest staying with a tube preamp to match with your SS amp. Sonic Frontiers Line 1 or 2 might be more inline on an older amp as you might be spending too much on a preamp and might not hear what it can really do. I have a Pass so I'm not talking down on the older Thresholds, just guessing you're not wanting to go crazy here. Plenty of nice tube preamps out there. I'm sure you're Blue Circle sounds nice, so you might be spending some to match or better what you already have.  I have the ModWright LS36.5 which is a great preamp but it only has one balanced input and output, so if you need more balanced inputs you'll need to look elsewhere. ModWright and Pass together is very musical. Would help to know what you're budget is.

Yes, for SS go with a Pass Labs!

The Threshold SA4e input impedance is only 600 ohms in balanced mode. If you want to run balanced you will most likely need a ss pre. I can't speak for the MW, but the ARC would have to used in single ended mode. It wouldn't be able to properly drive the 4e in balanced mode.

Yup, had an ARC years ago and due to impedance mis matching could not use the XLRs from the ARC to the Thresholds. I’d stick to using RCAs as the Thresholds are not differentially balanced. Still rock nonetheless.