Krell FPB 300 and Tube preamp pairing

I recently bought a pair of Bryson 7B-ST amplifiers locally and shipped them to my brother in California to drive his Magnepan 1.5QRs. I visited him a couple of weeks ago and had a chance to hear the system. We both thought the sound is on a bright side, actually too bright. Prior to the Brystons he was using a Sonograph SA250 and the sound quality was more acceptable. I felt kind of bad since I was the one who convinced him to change arguing that the Sonograph was underpowered for his speakers. His preamp is one of the top models from Adcom he bought at the same time he got the Sonograph and the Magnepans, around 20+ years ago. I initially thought maybe he should get a tube preamp to pair with the Brystons but we’re worried that may not cure the brightness. Making a long story short, he’s found a Krell FPB 300 locally and is also looking for a tube preamp to pair with the Krell to ensure no brightness and asked for my advice, again. I did a little research and found the following verbiage in the FPB user manual and wanted to ask how should I interpret this or just rule out the whole Krell idea altogether. He’s now very bright-shy and doesn’t want to take any chances and I completely understand it. I know the question around which amps play well with Magnepans has been asked and answered. I’m just asking about Krell FPB 300 sound signature and its pairing with tube preamp. Thanks and sorry about the long post.

"Tube preamplifiers, by design, are capacitively coupled. For this reason, the benefits of a direct coupled amplifier cannot be realized when used with a tube preamplifier. Additionally, many tube preamplifier output a great deal of DC. This DC may exceed the servos of a Full Power Balanced amplifier. Excessive DC level in a signal can damage amplifiers, speakers, or both."

Edited to ask if there are any suggestions to tame the brightness with a tube preamp that pairs well with the 7B-ST in the $1500 price range, new or used.