It's a bit of the right town, wrong street problem.
What makes the P-250 attractive for its intended use is its price / performance utility and decent midrange / high sound. That also makes it work for acoustic instruments, particularly with a DI box, but even OK without one. A lot of that comes from the same thing that makes it tough to get solid bass even with outboard EQ - many small speakers.
One reason why standard guitar amps sound horrible with acoustic instruments is that they are bass-heavy; large speakers pushing a lot of air with large amounts of speaker travel. It pumps out lower frequencies that make your D-18 sound like a swamp. That's why the P-250 works well; it is the antithesis of the monster speaker syndrome.
But - everything is a compromise here, particularly in this price range. What makes it good for one thing tends to handicap it when you're looking at something else. The Kustom system might have given you better bass, but don't expect as much from it as a well-rounded PA. The Passport is a performer for the money, but it wasn't designed to be a bass heavy system.
How about kicking this idea around for a bit - anyone experimented with powered subwoofers? I haven't. Can you run a line level signal from the P-250 tape outputs into a powered subwoofer, without any specialized EQ? Then let the big cone do what it does best? If you're getting some very low frequencies, the Passport speakers might not sound so anemic.
[This message has been edited by Treeline (edited 05-02-2000).]