Changed subs from speaker level to XLR after many years. Wow!

Connecting a Digital Drive using line-level RCA and speaker-level allows the sub to be used with a two channel and a home theater system, just not at the same time.

Connecting line-level RCA and speaker-level can aid in filling in the missing mid-bass in single systems with thin mid-bass. The Duel Connection Optimization procedure in the User's Manual is slightly different. 

Velodyne's professionally unreviewed Frequency Response and Parameters Screen in the redesigned DD Plus is on a whole other level of user friendly subwoofer specific Optimization than most any other low frequency processing subwoofer that I'm aware of. The DD connectivity flexibility is another notable feature. My two DD-12 Plus' are in their twelfth year of remote controlled fun. 

If you decide to add a second sub using XLR cables are perfect for very long runs wherever your rooms other bass mode location may be. Enjoy. 

While REL Acoustics (a favorite sub of many audiophiles) is a well-known proponent and champion of speaker-level connections, Brian Ding of Rythmik Audio---who also offers models with provisions for speaker-level hookup---prefers and recommends a line-level connection.

Interesting about Rhythmik’s suggestions.  One of the things I noticed yesterday was that turning on the subs (via XLR) caused even higher guitar notes to sound fuller and more in the room.  Listening to the quieter tracks of David Crosby’s if only I could remember my name, was probably the most realistic portrayal of live musicians I can remember hearing.  

I agree that the speaker level connection may not be optimal for all systems. IMO tube amps may benefit the most. Plus with Roon one has the ability to easily do a high pass on the main speakers allowing the sub to be dialed in at higher frequencies. 

This is interesting. I have dual DD-15’s in my 2 channel system and run speaker line level. Perhaps I will give the XLR a try.

If your subs have RCA inputs (and not XLRs) Jensen Transformers makes a wonderful product that does the conversion from balanced to single-ended. Its the ISOMAX SUB-1XR (you'll need two for stereo, or else the SUB-2XR if receiving stereo signals into one sub).

The transformers are optimized for bass and so have bandwidth to about 1Hz, sacrificing the highs (IIRC rolls off at about 7KHz).

Just took a trip over to Guitar center and picked up a pair of Mogami Gold 10 ft XLR cables. Spent an hour using the onboard EQ on each sub and listened to a couple records. I must say that the difference very noticeable. Much tighter and deeper. I had the subs set from 2 to 4 on the volume. Now they dialed in at 6 with a rather flat response at 60Hz crossover. It pays to keep an open mind!