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I recently ended up with a pair of TDL RTL2s for $100, seemed like a good deal, so I bought them. My other speakers are Royd Apex. Being that I have virtually no experience with audiophile matters, I have an idea for getting a more extended bass using the TDLs as a sub, looking for someone to tell me this is a stupid idea really. So the initial plan is to put a crossover in between the amp and speakers, so I can output frequencies below, say, 50hz to the TDLs and leave the Royds to do the rest. Has this been done before? If so, or not, would it work??

It can be done (and, no doubt, is being done somewhere) but it requires some really monstrous capacitors and coils, especially if one wants high quality.

The better way is to make the crossover between the preamp and amp(s). This line-level crossover is smaller and more flexible for lower cost and higher performance.

Kal

If both of your speakers are 8 ohm then putting a 25mH coil in series with your low frequency speaker and a 400uF capacitor in series with your upper frequency speaker will give you a 50 Hz 6db/octave crossover. Not that this is a good idea. Not familiar with your speakers, but do they really have much below 50Hz?
Using a crossover at the preamp out will require that you have two power amplifiers.

Cylclebrain, your idea doesn't take into account any efficiency imbalance between the speaker systems.

You'll have to decide whether you want a passive system or an active method of managing the crossover and whether you want one or two amps driving the systems. For the money spent, if you decide to go through with this scheme, I would suggest you buy a plate amplifier from any of the shops which specialize in DIY speakers. Just place "plate amplifier" in a search engine to find retailers. It will include the active crossover (hi/low or just low pass) with variable frequency selection, level controls to mate the two speaker systems together and an amplifier that will do well in the bass frequencies you want from the TDL's. Depending on the money you spend you can also find units with phase adjustment to compensate for placement issue with both speaker pairs in the same room. If you already have a second amp that can drive the TDL's, you can buy a stereo crossover with a subwoofer output from any pro audio shop.

If you were looking for this to be cheap, you probably should have saved your money on the used speakers.

You will have two complete speaker systems in the room. That isn't the most sensible thing for good sound or good decorating. And one plate amp will drive one speaker cabinet, though most of the plate amps can manage a parallel connection to both TDL's. If you only drive one TDL, you will undoubtedly have a level imbalance. And the TDL's were not the easiest speakers to drive to any reasonable volume so you might want to consider two plate amps.

Your idea is not completely far fetched but for the money, space and hassle invested, you would be better off buying a real subwoofer rather than jerry-rigging this combination. Resell the TDL's or move them to another room for a "B" system.

My response doesn't take into account alot of other things and was unrealistically simple. Do you know of a 8 ohm speaker that is a simple 8 ohm resistive load over any frequency range? Just giving a rough answer to someone that wants to experiment.