The 990 has an excellent bass management program which allows you to set crossovers for each speaker individually, so I would take advantage of that feature and use different points for each speaker. The EP600 has a remarkably flat FR and before starting with each speaker make sure you have found the location which produces the most linear in-room FR. If you have a RS metre, place the sub at your favourite seating position, runs sweeps and find the location along the walls where there is less volatility with the SPL readings.
In setting the crossover your intial guide should be the low range of FR -/+3db. For the M80s I suggest crossover at 1/2 octave over the lowest extension of +/-3db-- that would be 50hz (34hz x 1.5). For the Centre I would recommend 80hz--any higher male voices will sound too hollow and any lower there will be a gap in FR. For the QS8s, I would start at 100HZ and check at 110hz and 90hz.
I've always found the spaciousness and air of HT sound is much improved when you take full advantage of each speakers FR.
Ideally you have each speaker blend well with the sub at the crossover points without any gaps in response.