Most sub bass on most recordings is mono. If you use the proper crossover as I specified, there will not be a problem getting proper integration. And you can use stereo subwoofers, one set to either side of your listening chair, if you are worried about that issue. I am presuming you will crossover at around 80Hz. However, to do this right, will be a little costly. A minimum quality subwoofer that I would use for this woudl like the SVS entry level subwoofer or comparable(there are not many that are comparable for the price). So, one would be $400, and two, obviously $800. If you cross over at say 150Hz, then you would need two subwoofers. I would place each one on a short stand on either side of your chair. I would also add in a high density acoustic dampening material to the subwoofers - the stock absroption material will not be likely sufficient, as it would not be presumed you would use the sub to this high of a frequency. Then you need the crossover. You can use a low cost analog one that is $80(Behringer CX2310 -- don't let the price fool you - this is a very well built device using very high quality parts). But this is purely a crossover. Ideally, you would want a powerful DSP EQ as well. The Behringer DCX2496 is the answer that solution. It also has more powerful crossover functions to aid you in more precise crossover settings. But, the simple analog one would serve the purpose fine. You would of course need some sort of pre-amp, because you need to send a signal from your source to a volume control unit(preamp) that will feed the input of the crossover, then the crossover outputs feed the headphone amp and the subwoofer(s)."I wouldn't bother much about this, but just buy a good headphone"
I already have a pair of AKG K701s...
"You don't need the bass drivers near your ears unless you are attempting to blend in bass at a rather high frequency. For the normal band of subwoofer base, around 70-80Hz and under, a good quality normal subwoofer placed next to you plus an active crossover to roll off the headphones where the subwoofer starts to operate will do the trick. I suspect that the active crossover is important for optimal results. Now, if you intend to crossover at 120Hz or something, then the twin woofers located on either side of you may offer some added level of performance."
Well, the reason I want near ear drivers is that with a headphone, only the left ear receives sound from the left driver, and the right ear receives sound from only the right driver (part of the "sound" of cans). So I figure that using monaural bass would not mesh with the headphone sound.
Oh, I did not realize you were willing to build. In that case, yes, still roughly $800 total including the amplifier(s) to drive the subwoofers. Some extraordinary quality woofers I recommend are the Eclipse SW8200($220 each new on eBay w/shipping). They use the same motor technology as the LMS Ultra subwoofers, because TC Sounds designed these for Eclipse. Needless to say, these can produce some extraordinary quality bass. Use these each in a 3 cubic foot sealed cabinet and use the DCX to flatten the response(it is common technique to use subs in cabs too small and then correct with EQ, which the DCX will make easy). You must of course get the Behringer DCX2496 to do this correctly, assuming you get a stand alone amp. However, if you use plate amps; the O Audio 500 Watt BASH, one in each cabinet, then you could get away without the DCX because this specific plate amp has built in filters to correct the response, and it has sufficient power for your purposes, and use the low cost analog crossover I recommended earlier. However the DCX is still the superior crossover to use for ultimate control over your sound and I recommend it if you can fit it in your budget. I am more than happy to help you out with specific programming, set up and wire routing/adapters for the DCX to make this work optimally if you go that route.Erm, this is meant to be a DIY project.
Though if you WERE talking DIY, you meant 800 for two hand-built subs?