which sub to get

Does anybody else use something like the "high level" speaker output driven input like REL?

Velodyne. I had a Velodyne SPL 1000 about 10 years ago with speaker level inputs. The SPL series was replaced with the optimum series.

Don't rule out the Legacy Audio Subs. They make 4 models: the Goliath, (The ultra-high performance Goliath XD subwoofer from Legacy will match the dynamic range of even the most efficient speakers. Massive motors on dual 15" Silver/Graphite/Rohacell woofers provide world class performance and will handily deliver 130 dB of low distortion output. Bass pressure in rooms will rise and fall so quickly that room boom and droning is not an issue. Kettle drums will roll and thunder, kick drums will have visceral punch and pedal notes will surge with this Goliatath of a subwoofer.)
The LF Extreme (
The Xtreme XD has the power and speed to keep up with Whisper. Dynamic braking, a Rohacell/Silver/Graphite cone and additional ICEpower® amplification make this unit absolutely world class. Bass with BOUNCE, not bloaf.), the Point One HD, (The Point One subwoofer is one of very few subwoofers that can actually enhance the performance of the Focus HD speaker systems. Providing total control over level, phase, and crossover frequency, the Point One also boasts Legacy’s unique BLEND feature. Now with balanced inputs.), and the Metro, (As a 16" cube, Metro is the smallest of our renown subwoofers. Capable of an amazing 117dB of output, Metro is designed to augment systems with woofers 10" and smaller.)
I use two LF Extremes in my system. They blend in well with the Focus SE, the Helix, and I'd hope the V as well.