Can the KEF LS50 rock?

KEF LS50 is overrated and overpriced. The implementation distorts the sound quite a bit when compared to accurate speakers. Go with the ELAC floorstanders instead. (Another option is the JBL LSR305/8 + 310s sub in a 2.1 setup, that's what I have now, super accurate, super awesome, super value)

The LS50 got great resale value though, I sold it in one day for about the same price I bought it at.

Hi there Brimel
My son has the LS 50s with a nova 65 SE. In a medium size basement room - it plays plenty loud. Adding an REL T9 added some serious bass which can definitely be overdone. But yes they can rock, just not at deafening leveIs. I set up a system for friends consisting of Peachtree 200 separates and a pair of LS 50s with a T7 sub and they Absolutely love the system in a big living room with 8-9 foot ceilings and have no interest in upgrading anything. I set up another friend with the same Peachtree separates in a huge room with cathedral ceilings and selected Vandersteens 3s because the room was simply too big for LS 50s. He absolutely loves the set up. No interest in upgrading. I also have LS 50s paired with a Sprout that sound fantastic in a 14 x 12 bedroom. Sooo. In a medium sized room where you don't often play music very loud a 125 SE
paired with LS 50s should work really really well for you. Add the T5 you already have and you will get more bass that will be a welcome addition if you don't ask it to play at rock concert levels. REL subs don't relieve the load on either the Peachtree amp or the LS50s as the others have pointed out. But they will simply add more bass - which is a very satisfying addition to an already fantastically well reviewed and in my opinion wonderful speaker. 
The UltraFidelis website has a great explanation under the heading why everyone needs a good sub. But what they are talking about and the other gentleman here is talking about is adding filters between the pre and power amps which you cannot do with the 125 and is what Vandersteens subs and their higher dollar speakers do. Better?absolutely, but way more money. I'm guessing you will absolutely love the 125 with the LS 50 T5 combo for 95% of your listening. We have used anticables with bananas on each end with great success. Don't overthink it and enjoy the music!
FYI Main system is CJ PR12s PR16LS 2 Ayre QB9 DSD GYRO SE SME V Benz Ebony PR15
and B&W Signature 30s. 

I have had a ton of monitors through my system in the past 10 years and the LS50's were my least favorite, notwithstanding their fine reviews.  I found them to be harsh and very hard to drive.  I have fallen in love with Dali monitors, very neutral in their sound.  I have a REL T7 sub that fills out the LF very well with my current OB speakers.  I think some of the best monitors around are offered by Fritz Audio and would suggest you consider a demo of them.  No question that ELAC floor standers ought to be on your radar, particularly at their price point. 
Good luck, Whitestix