I've been thinking about picking up a modest powered sub for these speakers (ie: $150ish or less), something along the lines of a BIC V-1020 or Dayton SUB-100. I'm a bit confused at how it would properly connect in with my setup though.   My AV40s are currently connected to my LD MKIII tube...

I currently use a pair of K240 MKIIs hooked directly into a Xonar DX sound card (no amp). I only really use my headphones with my PC at home, the odd time I'll use them on a road trip with my laptop and Cowon D2 + FiiO E5 amp but not very often.   In the past I've owned Sennheiser PX100s...