tuck0411 and kwarny, thank you very much for the suggestions and information!
In my HT, my wife and I are the only audience and thus smoothing out the FR is not my primary concern. If I buy 2 F12SE subs, one will be placed on the floor while another one will sit on a large concrete windows sill 18" above the floor. Not sure if there is any adverse effect on the sound quality, but the wireless adapter from outlaw can solve my problem if both subs must be put on the floor. The F12 also rolls off at a higher frequency than the E15 to work better with my surround speakers, while a single E15 has better WAF than 2 F12s. Tough choice indeed.
The F15 and F15HP are bit tad too large for flexible placement in my situation, so I can only consider the F12SE x 2 and the E15. I guess the E15 should be very similar to the F15HP based on their specs.
At the beginning I was deciding between SVS/HSU/Epik/Elemental Designs/Rythmik/CHT so I spent the last couple of weeks reading those "vs threads" and shootout threads. And just reading these threads alone I have probably read over 1000+ posts. I already eliminated CHT/Epik/ED due to frequent complaints on their mediocre to poor product quality or customer service, and recently I eliminated SVS as well because of the non-performance of the local distributor which double the price tag (e.g. the $1100 SVS SB13+ is being sold at $2250 here after discounts) yet providing poor service. The only sales representative of SVS here is also outsourced to a call center in China who never actually see the products himself. I am just paying more for less.
This leaves HSU and Rythmik as my final candidates. HSU and Rythmik win people more praise here for the sound quality over SVS, and according to a few owners like Curtis who owned many different ID brands, the Rythmik should have a slight edge in sound quality. So I will either opt for the ULS-15, or the E15 / dual R12SEs.
No matter what happened, I think Rythmik should regularly respond to potential serious buyers' questions. HSU is top notch in customer support, but since I don't urgently need the subwoofers, I will give Rythmik or Brian a few more days to make up my mind.