$400 is a bit tough for pre-amp choices...I would go with a good stepped attenuator as has been suggested, if that's the highest you're willing to go. If you can stretch the budget a bit...a Mapletree tubed preamp (Line2A) is $550-ish to 600 range...I recommend it all the time. Aikido preamps are nice...if you can't build yourself, they're sometimes available used for fair prices. Next best I'd consider is probably the SAS preamp which is ~$750 or so used. Other options are upgrading the source and getting an external DAC that also works as a preamp. The TADAC is one such DAC...and a few others exist that do the same. Promitheus is supposed to be nice, as well, but I'm not sure about the wait time for a new unit.
Again...for <$400, stepped attenuators are great. The EVS ultimate attenuator is one of the best available...a pair run about $200 or so used.
http://mysite.verizon.net/vze4c5pt/id2.html Are also a great option...I own a pair and find them to be rather nice.
What I've suggested is based purely on sound quality and not convenience.