I don't think OP started this discussion for you guys to come in and bash his system or his choice of equipment, or for you to argue with each other. Really, this forum has become such a slug-fest it's unbelievable! If some of you actually knew as much as you think you know, you could save the world! You even have him believing that you know more than the engineers and designers at REL, who designed these subs!
OP has nice equipment! His choice of Mac is great, and it's not underpowered. 75 tube watts from a Mac is a lot of power. They use excellent transformers and excellent design. They will power those 86db speakers just fine. The RELs are good subs. No, they don't go to 10 Hz, but neither does the music!
Sure, a new amp, 4 big subs (or is it 5 now, MC?), new, more efficient speakers, and tweaky fuses, cones and power cords would make it sound different (not necessarily better). But, that's not what he asked. His question for you guys was whether or not he will see improvement by adding the RELs to his system. He isn't living in an anechoic chamber, and isn't looking for perfect, world class sound, and he isn't building a studio or trying to spend tens of thousands of dollars!
The answer to the question, @ricmci is YES! It will definitely improve things for you, and I think you have already discovered that. IME the RELs are very musical. They integrate very well with your speakers if you take your time in tuning them, and I recommend following the OE instructions to connect using the high inputs. Are they the best available? No. Are they junk? Far from it. In my case, I have Primaluna HP Integrated with Revel F208 speakers and a pair of REL t/9i subs. The SQ is fantastic!
Some of the forum regulars need to grow up. It's like OP came in asking if he should paint the walls blue. And, then this group gets hold of him and tries to convince him blue is wrong, and he should move walls, raise the roof, and move to Colorado where the power grid is more reliable. If he's happy with what he's got, then let him be happy! He is taking a journey, as we all are. Instead of encouraging his walk up the hill, you want him to go to the moon. Sheesh.