Small sub

It depends on your room size as much as anything. I use a Martin Logan Dynamo, which is a one foot cube and has a 10" driver. It works very well for two channel purposes, but it would never work in my HT setup, where I use the much larger ML Depth(i). There are many variables, i.e. crossover points, etc. that must be dealt with. I just run my Merlin monitors full range and have the Dynamo fill in underneath the Merlin's lowest rated frequency. This method works very well in MY system. However, if your goal is to use a crossover point above your monitors lowest frequency, the integration becomes much trickier. If this is your goal, I would suggest the velodyne subs (SPL class and higher) that have built in EQ.

I'll back up what Cruz123 has to say...

I have (2) ML Dynamos in my system and love them. They have a variable crossover point down to 30 Hz and can be run at speaker-level. I definately agree that these are not "HT Subs", but they work very well for 2-channel. The larger ML subs are nice, too, as well as REL subs.

REL acoustics q108e might be a good choice. I have one on order now from Gateway Sound in GA (they were great to work with!) to match with my Spendor s3/5r's. I will let you know how it works.

i will second the thiel recommendation. i use the first model, sw1 which is now called ss2 or something like that...tightest sub i have heard and unlimited so far for my needs...