Best Subwoofer for Magnepan speakers

Greetings Audiophiles ! I would like your imput. My present system is composed of the following: 1. Conrad Johnson PF-R Pre-amp. 2. McIntosh MC-300 Power Amp. 3. Magnepan 3.5 Speakers. As everyone already knows, the maggies have a great soundstage (with a powerhouse amp) but are somewhat weak on the bass. So do I want a subwoofer with a built in amp or do I want to power one with a dedicated amp? What brand subwoofer? Budget around $1500.00
I really value all your opinions! Thanks Kyle