IDSkoT 5,000+ posts
Resonating Reticular
That's going in my sig.i tune while I drift.
As for OP:
External processors (good ones) are expensive and work just as well as internal HU processors... Not to mention, it's more complicated to tune them since odds are it's gunna be in a weird place so it's hidden, thus making tuning a long and strenuous process.
Active HU's are much more feasible since you're sitting where you normally listen to music, so any adjustment you'll be able to hear, and you can continuously adjust it as you drive. Another option is to get active-ready amps. But, even then, these amps are costly...
I would sell your unit on here or eBay, and buy an active unit... the Alpine 9887 is popular, as is the Pioneer 800MP. I don't know of any other active units that have received stellar reviews.