Tight bass sub recommendations

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. 

If you are looking to keep your 1.7i for a while then I would concur with those who suggest making the investment in a pair of RELs.

It will not only deliver clean tight bass, it will also enhance your overall lower mid range by revealing more depth and texture within  your sound stage. 

I was looking to invest in a new pair of speakers a few years ago. I changed my view on how I was looking to achieve my goal of obtaining better sound. It was then that I realized that I could better than accomplish my goals if I invested in a pair of Subs. 

I looked at JL as well but found that the Rels were more Musical - especially using the  Neutrik SpeakOn connectors at the High Level input to the sub bass system. Although REL makes these cables, Three are a couple of other companies that make upgraded versions. 

I still have my Speakers and REL's and have no plans on upgrades..

If you go on the website there is a speaker - sub pairing section. They recommend the T-9I for your 1.7I.

Good Luck

Speaking of my favorite... REL S Series subs. There is a limited edition one with the driver from the G Series installed inside the REL S5 cabinet and more. It just received a WOW from Tone Audio. Tone Audio uses a pair of the REL S Series 212SEs as their sub of choice for their system since reviewing them. The REL 212SE is like having 2 REL S5s stacked on top of each other.
Anyway...... This review of the REL Carbon Limited gives you more insight into why some of us audiophiles find no other choice for a subwoofer (pair needed) than REL. It's the "REL EFFECT"
Here is the link...... http://www.tonepublications.com/review/the-rel-carbon-limited-subwoofer/

Any sub that does not have variable slopes, 0°->180° continuous phase adjustment and phase invert should not be considered. Unless one is  very lucky, getting good integration sans these may be difficult.

Many claim to have a great setup, but if what I hear at dealers in any indication of what people have in their homes, I'll have to reserve judgement. Most dealer setups are universally awful.

After 15 years, I replaced my [customized] ACI Force with a pair of Martin Logan Dynamo 800x. The best location for single sub in our media room is in midway between the mains and halfway to the listening position. Two subs definitely broadens and focuses the sound stage.

The ARC software does a good job of controlling room defects. I'd recommend getting the Perfect Bass Kit as it does a better job than using a smartphone microphone, calibrated or not.

The smartphone control app is a dream. After decades of knob-dicking, zeroing to the best setting is a doddle. Raise the XO 5Hz, shift phase -3°, drop level by 1db, shelf the bottom octave -1db. It's so easy, it's silly.

Only negative, and it applies to most modern small subs, is they are too light. My 15 year old Force outweighs the 800x by 50% for the same driver size, amp power and performance range. On a large 2nd story wood floor, strolling may be a problem, especially with the rubber feet. Easily resolved with 10kg barbell weight with an added side benefit of MUCH TIGHER bass and far fewer room resonances. Strolling is less of an issue when using the spikes. [What's 8 more dimples in the bamboo?]