Everything in audio is about synergy between components. Having owned Nak cassette decks but not their separates, I can’t comment on the authenticity of his remarks, other than to say - the Nak preamps were built to augment and work with their power amps. Why would any manufacturer of audio gear that offers amps and preamps design them not to work well together?
I would ask him what else he is using in his setup. Maybe his speakers are poorly placed, too high up in the air and therefore, not giving enough bass? Maybe there is something else in the audio chain, like ICs or speaker cable that prohibit bass or perhaps the amp he’s using with the CA7 doesn’t produce a lot of bass?
I appreciate the sellers honesty, I just think that finding out what else is in his system may lead you to why he made that remark about the preamp not having bass. Also, perhaps there’s something wrong with the preamp too?
Personally, I like the more minimalist CA5 preamp over the CA7; however, if I owned the PA7 or PA5, I too would be looking to partner it with a Nak preamp as well.
I just realized he has the PA7A preamp. I think it also is a very good idea to research Nak preamps. I think the earlier CA5 and CA7 may be better than the later 5A and 7A, not sure? He may have just done you a huge favor not selling it to you? BTW, what is your preamp budget?