I'm in Bangkok.
Well, I'm in Thailand & toastiness aside, they are good, but not excellent headphones. The bass is there, but it's definitely lacking. I was a little disappointed.
I also have the Sennheiser 660S and after considering for a long time of adding a subwoofer, I finally tried it and the results are amazing. I am using the SVS 3000 Micro, which is quite small but powerful with two drivers (both active, not passive).
The SVS app that runs on iOS or Android makes it easy to control the sub remotely via Bluetooth, even though the sub is right next to my chair and up off the ground a bit on a small table. The SVS sub has extensive DSP processing so that one can tailor volume, crossover frequency, degree of crossover slope per octave (3 dB, 6 dB, 12 dB, etc), phase, polarity, and has a full parametric equalizer on top of that.
I could have connected the sub to the pre-outs on my Schiit Magnius headphone amp (which would control the volume of my sub along with the headphone volume), but I decided to connect it to the RCA outputs of the DAC, which means I have to control the sub volume from the SVS app, although I find it is not necessary to change the volume very often. My iFi Zen Signature 2 DAC has both RCA and balanced outputs, and the balanced output goes into the balanced input of the Schiit Magnius headphone amp. The main reason I did this was that I read that the SVS DSP processing introduces some time delay to the signal, so connecting the sub directly to the DAC might make it in better synch with the headphone connected tot the headphone amp (assuming that the headphone amp has some small amount of time delay also). But not sure that there is an audible difference if both the sub and headphone were connected to the headphone amp outputs (line level pre-out for the sub, and high level headphone out for headphone).
As I said above, the sound of the HD-660S and SVS 300 Micro sub is amazing when used together, and it resembles the sound I have heard with flat panel or electrostatic speaker systems with built-in (or external) subwoofers. There is no hint that the low bass sounds from the subwoofer are coming from anything other than the headphones, even though they obviously are coming from the external subwoofer. The main downside is that the SVS 3000 Micro costs $900 USD, although it is cheaper than the KEF KC62 micro sub ($1,500 USD), even though the KEF does not have remote control. I think a larger and cheaper sub-woofer like those on sale at Emotiva might also work if one can place it so that one can control the sub from the listening position.