I'm sure the TP22 will be loud enough to fill the room with some sound with the RB-51s
The RB-51s are rated 50Hz-24KHz ± 3dB, which means you would probably set the crossover on an external sub to somewhere around 50hz to 55hz to integrate the sub (although if you like a different setting, go with that--let your ears be the judge). With the Promedia amp, you aren't going to hear any midbass from those speakers. It's almost all coming from the Promedia sub. I would bet that the RB-51s would have better sounding midbass than that Promedia sub (I used to own the Promedia 2.1).
When you get ready to buy a sub, I recommend researching and asking questions in the AVS budget subwoofer discussion thread. There are people there that know a lot about home audio subs that are constantly watching for best deals.
And I would look for a sub like the Dayton SUB1000 that has high level speaker level inputs AND outputs:
That allows you to run the speaker out from the TP22 to the speaker in on the sub amp, and then the speaker out on the sub amp to the speakers. That's a passthrough connection (the sub isn't powering the speaker; the TP22 would be), and the advantage of it is that you can then set the gain (volume) on the sub to match up with the speakers and use the volume control on the TP22 to adjust the sound. You could get a sub that has left/right RCA inputs only, and split the sound coming from your computer and send it to both the sub and the TP22. Only then you would need to set the volume on the TP22 and the gain on the sub to match for good integration, and from then on use the volume on your computer to control the sound level.
Yea the sub-800/1000/1200 are definitely next on my list of upgrades. just a little strapped for cash atm so im holding off and using my old promedia sub to power the speakers till the tp22 arrives. You think that the rb-51s will sound better without the subwoofer as whole (again in the split connection setup not that speakers coming out of the promedia sub setup)?