Stacking subs to save space

Some subs are designed to securely stack (eg, some higher end RELs) but for those at aren’t I’d worry potentially about shifting due to vibration, as one potential downside. 

Stacking them will permit them to couple and slightly increase output.  OTOH, they will both be subject to the same room-boundary effects, effectively increasing the magnitude of those peaks and nulls.

I'd prefer them separated and placed asymmetrically (and EQ-ed).

I would also think that the vibrations from the top sub would affect the bottom and or vise versa. Anyway to place them side by side with a separation between them?

ozzy

You can probably use some feet or material to make sure vibrations don't cause the top sub to shift off the bottom one, and I would assume the existing feet are sufficient to deal with most of the vibrations.

The downside is really the loss of smoothing out the bass response due to having both subs in one location, as kr4 mentioned.

Hello Steve,

     For best bass sound quality performance, I'd suggest you do not stack your subs.  Your pair of Rel R3 SHO are high quality subs.  However, they need to be positioned in the room and configured properly for optimum performance.  
     By positioned properly, I'm referring to each sub's room position in relation to your listening position.  Proper positioning does NOT mean just placing one Rel sub next to your left speaker and the other next to your right speaker.  If you do some internet searching, you'll find there are a boatload of recommendations on the best way to position and configure a pair of subs in a room, both with running them in mono and stereo modes.  
    Based on my experience using multiple subs in my systems, I have developed strong opinions on which mode is best but I don't want to detail my opinions right now since, as far as I can discern, it seems your Rel R3 SHO subs are only able to be set-up in a stereo mode.
     So, I think you should probably restrict your research to stereo sub set-ups for the time being.  I believe your best chance of attaining very good bass performance in your system and room, and gain very useful experience along the way, is to experiment and tryout as many stereo sub positioning recommendations as you're willing and can until you discover one that you prefer the most. 
      I know Rel suggests positioning a sub angled diagonally outward in each front corner of the room behind your main speakers.  I'll start it off with my alternative suggestion but I think these are just two worth a try. 
     I recommend you use the 'crawl' method to best position the right channel Rel sub on the right side of your room, which could be anywhere from the right side of your tv stand along your front wall, along your right side wall to the right half side of your rear wall.  And then use the 'crawl' method to best position the left channel Rel sub on the left side of your room, which could be anywhere from the left side of your tv stand along your front wall, along your left side wall to the left half side of your rear wall.
   
The best procedure I'm aware of for doing this is called the 'crawl' method. Here's the positioning procedure:

1. Hook up the first Rel right channel 'sub R' and place it at your designated listening position.  Set the Volume control at the 12:00 position (50% volume), the Cutoff Frequency control at 40 Hz and the Phase control at in-phase ("0") on.sub R.  Play some music with good and repetitive bass.
2. Starting at the right side of your tv stand, crawl slowly on your hands and knees along the front wall  listening for an exact spot where the bass sounds best to you(detailed, smooth, not over or under emphasized and natural). If you don't find a good spot along the front wall, continue crawling around the perimeter of the right half side of your room, in a clockwise direction, until you find an exact spot where the bass sounds best to you.
3.  Once you identify the exact spot where the bass sounds best to you, move the Rel subR from your listening chair position and place it at this spot.
4. Hook up the second Rel left channel 'subL' and place it at your listening position.  Set the Volume control at the 12:00 position (50% volume), the Cutoff Frequency control at 40 Hz and the Phase control at in-phase ("0").   Play some music with good and repetitive bass again. 
5. Starting at the left side of your tv stand, crawl slowly on your hands and knees along the front wall listening for an exact spot where the bass sounds best to you(detailed, smooth, not over or under emphasized and natural). If you don't find a good spot along the front wall, continue crawling around the perimeter of the left half side of your room, in a counter-clockwise direction, until you find an exact spot where the bass sounds best to you.
6.  Once you identify the next exact spot where the bass sounds best to you, move the Rel subL from your listening chair position and place it at this spot.
7.  Play some music with good repetitive bass one more time.  Sit at your designated listening seat and, with both subs active, verify that the bass sounds good to you at your listening seat.  If it does, you've completed the proper positioning process.  If not, you'll need to repeat the procedure, starting from step 1.

     The next important guidelines and procedures are configuring each sub properly, this procedure involves the setting of the Phase Orientation, Orientation (of the sub) and Crossover and Level Settings on each sub for optimum bass performance.  I recommend just referring to page number 15 of your Rel's Owner's Manual and following their configuring guidelines and procedures for the following:


3. Phase Orientation: 
5. Orientation:
6.  Crossover and Level Settings: This is copied from page 15 of your Rel's owner's manual just to give you an idea of what these guidelines and procedures consist of:  To determine the crossover setting, take the volume of the Rel (using the HI/Lo Level control) all the way down and put the crossover to 30 Hz.  At this point, bring the Rel's volume back up slowly to the point you have achieved a subtle balance, i.e. the point at which you can hear the Rel playing even with the main speakers playing.  Now, bring the crossover frequency up until it is obviously too high; at this point bring it down to the appropriate lower setting.  For all intents and purposes, this is the correct crossover point.  Once this stage has been reached, subtle changes to volume and crossover can be accomplished to provide the last bit of complete and seamless integration.  With that, set-up is completed. 

     Here's a link to some other recommendations:
https://www.aperionaudio.com/blogs/aperion-audio-blog/dual-subwoofer-placement-for-your-home

Good luck,
   Tim

Listen both stacked and apart. What sounds better to you?