Have a pair of REL S3 HO and they don't produce enough bass

Hi all-

Just picked up a pair of REL S3 HO subs to work with both my 2 channel system and my home theater. The high output speaker connections on the subs go to my 2 channel system and plug into each mono block amp. The LFE connection from each sub hooks into my Anthem 720 receiver for home theater. 

For the 2 channel system it seems to work well. Nice low bass helping out my Focal Kanta No. 2 speakers. 

However, for the home theater system, they just don't produce enough bass. I have to turn them ALL the way up using the sub's LFE volume control, AND turn them all the way up in my Anthem receiver settings to get any semblance of home theater bass. I know this isn't normal, so something has to be either wrong, or the subs just aren't meant for home theater use.

I was using a pair of SVS 12" subs and man those things rocked. However, I picked up the RELs so I could integrate them with my 2 channel system as well, something the SVS' could not do.

Any idea what could be the problem? 

A side note: I have NOT run Anthem's ARC program again since buying the RELs. Perhaps that could be the issue, but I didn't think it would make that much of a difference. However, I could obviously be wrong. :-)

you answered your own question rerun the arc.
They are probably less efficient than your svs and/or input voltage might need to be a little higher to get same sound levels.
generally ported sub produce more bass even with smaller amplifier power...Since S3 SHO is a sealed sub...it required more power to move the cone to produce equivalent bass that you experienced with your SVS ported sub...
What aawcy stated is absolutely correct. Ported subwoofers naturally will be more flat all the way down to 20-30hz. Sealed subs cannot naturally do the low frequencies because the frequency response curve drops off very quickly. The sealed subs require extended EQ to boost the very low frequencies to be able to produce the low bass you need.

You did not indicate which model SVS subwoofer you had. They have cheap sealed subs all the way up to huge powered ported subs. Your new REL subwoofers are probably a significantly higher sound quality and will have much better definition for music purposes than the SVS, but if you had a larger ported SVS, you might just have to accept the fact that the REL subs are not going to "shake the walls" like a big SVS. For home theater, there is no substitute for the big SVS ability to push air.
One additional question.  Are you connecting the REL subs in the exact same way you connected the SVS?  If you did a change in configuration, it could affect the way your Anthem routes bass frequencies.  Also, I would agree with batman's suggestion to re-run ARC.
No fix yet for this. I ran ARC again and same thing. I spoke with Rel and they suggested starting over again. So I’m just living with it for now. It’s adequate i suppose, but it’s not “great” which it should be. 
The REL S3 is designed for music. REL designs subs specifically for HT. In my opinion JL Audio subs work better when used for both music and HT. SVS is very good for HT.
Play with the HI/LO level adjustment...1 click at a time. I can quickly fill my room with more bass than my neighbors can stand with 1 S5 SHO. It does take some time and tweaking to get it dialed in but the results are worth it.
Play with the HI/LO level adjustment...1 click at a time. I can quickly fill my room with more bass than my neighbors can stand with 1 S5 SHO. It does take some time and tweaking to get it dialed in but the results are worth it.

The problem is that I'm only connecting the LFE input to the sub from the Anthem receiver, so there's really only one adjustment and that's the LFE volume. 

I've been working on this off and on since I originally posted this, and I've gotten nowhere. I called the shop I bought them from and asked if they thought trading them both in for one S812 would help and he said probably not. He thinks it's something with the Anthem setup that's not delivering full power to the REL. 

I don't doubt that, but I've poured over the Anthem setup and can't find anything that I haven't tried already. I turned ARC off/on, changed the crossover point...nothing changes the bass level. I should be getting blow away with the LFE volume cranked to the max...but just very OK HT bass. 

Not sure where else to go cuz my 2 channel sounds amazing with the REL sub! 

Did you have any conclusion to this?  I also had the same problem with an S3 non-sho version. At max volume it barely output anything with my receiver maxed out. Even though I had a T9 that could be run 1/4 of the way up and it was louder already. I thought the S3 was defective but when Rel got it, they measured that everything was to spec. So definitely there is something fishy. In any case, I ended up using the high low level inputs instead of the 0.1lfe input and now it has enough gain to be used for HT. However, I can’t use the speakon in parallel this way so can only use it as a dedicated HT sub this way.