Enhancing low frequencies for my system

Mike - it seems to me that you should have all the bass you need with the two Vandersteen 2WQ subs.  If you don't, I think you should be focusing on setup issues.  Setup elements could include:

- Location of the subs
- Setting on the subs
- Making sure the subs are not opposite polarity
- Which Vandersteen Xover modules you are using
- Room acoustics and sitting position

I would not spend money on more equipment.  You might be better off hiring someone who knows acoustics and subwoofer setup to come out to your place and spend a few hours tuning the system. 

Also, based on other posts, I think your McCormick DNA 0.5 Rev A amp should not be straining to power the Vandy 3a-sigs, since the crossovers relieve the amplifier of having to push much in the way of bass.  

BTW - full disclosure - the OP is my cousin Mike!  I guess I could have just picked up the phone!

I would not spend more money on more equipment: +1.

Have you paid attention to cabling, vibration control of components,
subs and speakers?

Absolutely, room acoustics are likely involved.
What happens when you move your speakers more towards
the rear wall?  Moving your seating position forward can
"take out some of the room," reducing the influence of 
the room on what you hear.

Do you have a lot of rugs, big sofas, other speakers in
the room?  Try "less is more" approach.

Reread "Get Better Sound" by Jim Smith?

I have to agree. I had a set of 3a Sigs for a while and they had amazingly tight and plentiful bass, nearly as deep as my Velodyne ULD-12s (pair) and “tighter” sounding. Richard Vandersteen is a bit of a bass whore, and with the 3a + twin 2wqs, you should have plenty.

I agree with look at your setup. Either you equiptment is set up wrong, you need to move your sub or seating position, or your room has severe acoustical issues. Regardless, a change in equiptment is not going to fix it (unless you are looking to do 120+ db at 20hz, then you will need some huge subs like Seaton Sound).  

Bass Traps
DSP based EQ
Placement

These are a few of my favorite things. 

OP, I would contact a Vandy dealer and have him/her check your setup.
If you are near NYC, Johnny Rutan would be the man to ask.
I owned the 3a sigs, and the 2w subs and while I never needed super bass, they always sounded full.
Bob

Isolate the source, obviously. Unless you have something against slam and bass extension.

Mike, your story sounds very familiar until I acquired my first Digital Drive-18 subwoofer with remote control gain and six preset EQd adjustment settings from flat, reversed polarity to however you want your low end to sound at the push of a single button.

The issue is usually the content of the recording. There are many albums were the polarity changes from track to track. Having independent on the fly control of the frequencies below 200 Hz kills the set it and forget it way of subwoofer ownership. Unlike the separate preamplifier idea your making these adjustment from the listening position that you optimized the system in the first place. 

I sold the 18 and now have two DD Plus 12s. The flexibility and the room lock is a wonder.   

Do you have access to a decent laptop?  One that you could install software on, and one that has a good-quality USB port?

If so, I'd recommend you download REW (Room EQ Wizard), which you will find at: https://www.roomeqwizard.com/  I would also recommend that you purchase a calibrated microphone, I bought this one, I think it's available from Amazon: https://www.minidsp.com/products/acoustic-measurement/umik-1

Having that combination helped me position and tune my subwoofers, as well helped me position and install bass traps in my "man cave", which had terrible low frequency problems, adding a "single note" to the bass, regardless of what the source material contained.

Good luck!

Your comments are great.Extremely helpful!Thanks so much. Mike