Fixed sub crossover on AVR...

Im contemplating buying an older but decent Marantz sr 5200 avr but the sub crossover is fixed at this going to be a major issue since my mains are full range? I could go the other route and use small speakers that go to around 90hz but how seamless would this be? I dont think the crossover is defeatable
Assuming the receiver has preouts, you could tell it you have no sub, the left and right speakers are large and then run the left and right preamp outs to the sub and use its low pass filter.

I don't see any problem with high passing your main speakers at 100 Hz except it could make the sub easier to localize than a lower frequency.