I have a medium sized room 9feet ceiling with 18 by 20 feet room. That room also extends into the kitchen and adds another 15 feet to the 20 feet portion of the room. Will LRSPlus with a sub woofer be sufficient or should I go for 1.7i maggies?
Definitely go with the 1.7i. A buddy has a room smaller than yours and the 1.7i is perfect. The speaker selection is the easiest part of your question, the sub is the hardest part. I'll explain.
The first time I heard a REL subwoofer was in July 2013, the sub was next to a pair of Magnepan 3.7 speakers that were driven by a Krell Full Power Balanced 200 (Class A) amp. The subwoofer was the REL Gibraltar G2 and it integrated perfectly with the Maggies, this sub was fast and tight without a hint of overhang all the while going down to 20 Hz, I instantly knew I had to have one. I finally bought a used REL G2 in April 2019 from The Music Room (yes, it took a while to save up).
Fast forward to October, 2023. The REL G1 MkII was on sale at a closeout price. The G1 MkII was being discontinued and this was the last time to buy a G1 MkII with full factory warranty at a discounted price. I bought one. I was happy with the Gibraltar G2 but the 10" driver wasn't too happy being used as a home theater sub. In some ways I preferred the G2 to the G1 MkII when listening to music because it was a little bit faster.
Now for the fun part.
In December, 2022 a friend needed a 2nd pair of ears to fine tune his stereo which consisted of Magnepan 1.7i and a REL S/510 sub woofer The S/510 replaced a REL T7/X which is -6dB at 30Hz, that sub didn't cut it at all with the Maggies which need bass to 20 Hz to balance the clear highs. The amp was a Pass First Watt F6, OMG what a combination to die for.
The Maggies with the REL S/510 had seamless bass to 20 Hz. Yes, the Maggies absolutely need a sub and preferably a REL.
I didn't need two subs and my buddy was curious to hear how my old G2 compared to his newish REL S/510 sub that also has a 10" driver plus a 12" passive radiator with 500 watts of power. We rolled the G2 into his listening room and tucked it behind his Magnepan 1.7i speakers driven by a Pass Labs XA-25 Class A amp and let 'er rip.
A minute hadn't gone by when my friend said "
my S/510 doesn't do THAT!"
The old G2 blew the S/510 out of the water in every single metric: extension, quickness, tunefulness, and the ability to blend in seamlessly and keep up with the Maggies. The DAC we used is the excellent ANK 5.1 and the system is music only so the home theater limitations do not apply.
If you have a medium sized room and need to crossover around 40Hz - 45-Hz then the G2 is perfect. Good luck finding one (and the driver will need to be eventually replaced for $800, it has a design flaw). The REL G2 ate the S/510's lunch.
After we unloaded the G2, even before it was plugged in, I put the furniture dolly back in my car because I just knew my REL G2 had found a new home.
For a room as large as yours a S/812 would be the minimum size and you might need two. The Carbon Special's would be perfect for speed and extension as would a used REL G1 MkII. Don't get the older original G1. The driver has the same design flaw, it is slower and nothing like the G1 MkII. REL has a Facebook forum that is very active. REL Customer Support posts all the time with answers. Hang out there for a while too and ask the experts about subs.
My full REL Gibraltar G2 and G1 MkII review
https://www.audiocircle.com/index.php?topic=187300.msg1963493#msg1963493
