I have used a single Rel 505 with the original pristine 2 with reasonable phase and integration success.
I updated my panels to the bass focused panels and the backplates to the max extent possible with the physical constraints of the enclosure, including a toroid 1, new bias module, caps and inserted a fixed resistor instead of the tweeter potentiometer. All had a marked improvement in performance, particularly in the bass.
now, I find the Rel sub is a challenge to get a correct phase balance with the new panels. The Rel adds base extension and midrange fullness to the new panels, but with the negative effect of precise imaging and some bass incoherences, likely from phase impacts for a slower driver and rear placement and some bass cancellation or phase effects from the dipole bass in the room.
I have tried to get the best fit in my small room for over a year timeline,
but to no avail . there is always a negative impact on the overall image presentation with the sub on, and some improvement in the bass extension and midrange fullness with the sub on. a real mixed bag, I can’t get the full benefits of both set ups.
I set the Rel crossover quite low, at close to the minimum, which I estimate at 30-35 hz. Any higher setting and the effects muddy the upper bass. It just doesn’t sound as open, or transparent with the sub in the chain.
Lastly, I found that placing the sub behind the panels created bass cancelation, which was immediately audible. I put in 180 phase shift which improved the bass performance, but still had what I felt was phase imbalance. I resolved to move the sub between the panels, somewhat out from the front wall to minimize phase impacts and adjusted output to just match the panel output for a smooth volume integration. Still, at the end of the day, it compromises the apparent speed and imaging of the bass focused panels. It does add deep bass extension and drive, but with an imaging and transparency price.
So, I switch it on for some music, but it goes off for critical listening.
Just some comments from my experience. I’m never done with tweaking the system, so the adventure continues…!