Tight bass sub recommendations

Another vote for JL Audio. Owner of two E110’s here. I honestly can’t see how anyone couldn’t like these subs.

I would recommend that you take a listen to the newest iteration of the MartinLogan Dynamo subs.  I am using a pair of the Dynamo 1100X's with my Magneplanar 3.7i's and they integrate very well.  They have the advantage of Anthem Room Correction built in.  As well, all of the settings can be changed from the listening position with an app that will run on an iOS or Android device.  In my system the Dynamos replaced a pair of REL T-9's.

LarryRS

I'm happy with using just one B&W ASW825,  which has an ultra-rigid. Rohacell® sandwich cone at 12 inches.  At one point,  I had 2 of them,  but ended up deciding to use the second one for my little recording studio and just one for my stereo.  It's a non-ported design and sounds nice and tight,  much quicker than it's bigger brother,  the ASW855,  which uses a 15" driver.
bwgroupsupport.com/downloads/specsheets/bw/ASW825%20InfoSheet.pdf

I have Magnapan 3.7s.  I use two REL G2.  Great match.  The REL was the first sub I had heard that has the speed to work with panels.  
Just my $0.02

Find some good ENTEC subwoofers, preferably refurbished.