Re: XTR Pro vs. P2 vs. Diamond M5
Either get a new mono amp, or get a pair of subs to play on each bridged channel on your current amp. I'd recommend the Sundown 8, or the SKAR 8, or the Sound Splinter 8, or the TC Sounds 8. All much much better than anything you listed above. A bit pricier? Yes. But still on the low end of costs for a good quality sub. Will they get down low? You bet.
The absolute best thing you could do is save up for another month or 2 to raise your budget for something a bit less pathetic.
2009 Hyundai Elantra - Budget Sound in Progress
HU- Kenwood eXcelon KDC-X994
Fronts- Crescendo MP6 mids, Vifa Ring Radiator tweets on Arc KS300.4
Subs- Single SA-10 on Mmats 2000.1 @ 1ohm
Misc- 1/0 CCA from AT, Big 3 in Stinger HPM, and AT 60 mil deadener
Bought from: Inferno333, select127, treesive, Kangaroux, DonH
Sold to: coolest user, Psychodrama, MidgetClown, mr.tigger, av83