Subwoofer with Thiel 2.4s?

This may seem like an odd choice but I would go with Vandersteen if it is for music. If you look at the design of both speakers, they are similar in many ways. I just listen to music and I have a pair that work very well for me.

These speakers have a -3db FR bottom at 33 hz. That is pretty low. That presents a couple of problems for using a sub. You would have to (I would anyway) drive the Thiels full range so balance wasn't screwed up (let alone signal degradation) by putting it through a crossover to roll off the bottom end to accommodate a sub. Next I would want a sub that was flat at 20hz or close, think big and expensive, and one which had a sharp roll off starting at about 32hz so the sub didn't extend up into the mid base.

Which leads me to suggest that you insure that you have already maximized the bass potential of the Thiels by careful use of boundary reinforcement (use a SPL meter and test disc) by changing speaker placement AND the listening chair. Both play a large roll in perceived bass.

I've only one been successful once in making a good sonic match with speakers and sub's (I like 2 more than 1) and even then I had to cede that the crossovers did cause the loss of a bit of resolution in the main speakers. It was worth it only because the sub was substantially better for overall bass and the cross over was sharp at 200hz for the subs and gentle for the mains.

Here's what I would recommend (and what I do with my CS 2.4's).... get a good subwoofer (I have a pair of Epik Legend) and only use them with the movies/tv. The 2.4's do have enough low-end that you're not really missing anything and adding a subwoofer and getting it perfectly integrated is not easy and very expensive (the subwoofer would be magnitudes of that you're willing to spend).

Used RELs are a often a steal, and it mystifies me why anyone wouldn't be able to dial them in to fit with most speakers. Level (the most important knob IMO), frequency point, phase, etc., should allow for just about anybody's taste for bass needs. I have heard that if you have bass heavy main speakers you can risk a wallow in some mud, but in that case just remember the level knob, or move the damn thing around...my older, bought used ($200!) Q150e is astonishing...I stand astonished.

Some of these details are in a couple amp threads that I initiated as I worked to build this system, but my real issue is that I am currently driving the 2.4s with a Pass Labs INT-30A. This combination, while sounding drop dead gorgeous with the right material falls flat on some more complicated stuff (e.g., rap, dance, & movies). I've tried another higher power amplifier (a 400 W Mac 402). This amp sounds great and solved the bass problem, but doesn't image quite as well as the Pass (or reveal as many recording details). I'm also considering a Pass X-250.5, but am still concerned that I'm going to end up missing what the XA amp gives me. It's just so good at imaging (which is the most important sound quality for me) and sounds so natural and grain free that I really don't want to lose it. I know the sub route is a cheating solution, but I'm on a budget and was just hoping that a little augmentation may let me fill in the low end without losing all of the positive qualities of the Pass XA series.

Thanks for the thoughts.