I am currently using a Velodyne DLS-4000R (400W, bottoms out at 25Hz) with my HT system. Can I use the EP500/600/800 as a second sub or are the quality/class differences too great to blend well? I read Alan's article about using a second sub and it said that mixing brands is fine but they should be of the same quality.
Thanks in advance.

I think you'd be fine.

Unless you have an SMS-1 or some other means of electronically calibrating multiple subs, you're going to be experimenting A LOT with levels, placement, phase, etc. anyway. My opinion is that the ROOM is going to make much more of a difference for you than performance differences between the subs.

You will find this article helpful on choosing and setting up multiple subwoofer systems.

http://www.audioholics.com/tweaks/get-good-bass/subwoofer-connection-guide

Thanks guys. I'll have to read that article once the web browsing filters at work have been turned off.

That sucks. Either you like a LOT of bass or your Velodyne is getting long in the tooth. My EP350 has never bottomed out and it hits 20Hz in my room.

I would definately add a second sub.

I tried the DLS-5000R during my search for a sub and found it to be very boomy. I suspect with the right amount of tweaking the 4000R and one of Axioms subs could play nice together, conversely you could find the velodyne just muddying up the sound. Either way getting a new sub sounds like a great idea and then try the velo out with what ever you buy and then run the system with the best sound set up.