REL S510 or S812

Im very happy with bass out of those speakers, but that being said i still find that even my HT 1205 turned up just a few clicks helps "fill" things out a bit. And if we are feelin like some rap, techno, or edm, ill turn it up a couple more just for fun. 

As far as room accustics, i have all that the wife will allow.  My profile pictures are not current. I went through the proccess of placing my speakers where bass and sound stage seamed best to my novice ears. I marked their spots and then I had a local dealer come out and do the ARC(Anthem room correction) process and it really improved things.  To my ears my system is sounding very good especially considering the room dynamics. The reason I want to add a better sub is to improve the overall SQ of my 2 channel set up. REL and others claim to improve mid and high end as well as sound stage.  At the same time i do want a bit more boom for movie time. The HT1205 was all that budget allowed when I began this journey. I feel its the week link at this time so time for an upgrade. As stated before, I still may use it as a second sub for LFE

I just set up a S/510 for my Dad.   Its a really nice sub.  I have an older r305 which has been rock solid , on for the past 15 years or so... continuously. 

The S/510 sounds excellent with a 2 channel source.  I have it crossed over around 60 Hz and have his Sonus Faber Electa  Amator III running full range.   Sounds phenomenal.    

In larger rooms a S/812 might be a better choice but I was really surprised at the output and how it sounds.  

The great thing about these subs is the speaker or line level connections can be "mixed" with the .1 LFE input.   This is such a great and important feature for those using a sub in a mixed 2 ch and surround set up.  I can not emphasize how important that is in achieving balance between the two.  

Thanks.  Thats 1 for REL.  What oddiofyl stated is really the main reason im considering REL

You might also want to look into the HSU VTF-3 MK5 HP Subwoofer

It works very well when used with a 2 channel set up and also if used in a HT set up.

Ill take a look. Thanks