My REL r305 is very similar and I’m running it speaker level from a 40 watt Integrated amp and I only have the volume 5 clicks up from zero and a low crossover and it is plenty loud and keeps up with a pair of 88dB sens speakers easily. Just a few clicks up and it is way too loud. 12x 14 room , well treated
it almost sounds like it’s not getting enough signal from the amp For troubleshooting purposes I would strip it down to its most basic elements, no EQ Just the amp, speakers, sub , and a good line level source