sub connection via high level output -> sound quality

Subwoofers with the level of adjustability afforded by SVS benefit from the direct output of the preamplifier. The inclusion of speaker level connections are a convenience feature for your exact need. 

In your case your preamplifiers signal will pass through another input / output stage of the  internal amplifier. Any low frequency limitations by the amplifier also limit the signal to the subwoofer. 

Not to fret. Once the subwoofer is setup you should instantly notice the benefit of the subwoofer regardless of any minor limitation. 

I would ask about your choice of the PB ported sub over the SB sealed?

@carlsbad If the sub offers this you can put it in series, yes. Maybe it´s me but I couldn´t find any REL that offers serial wiring.

@m-db I´m tending to go for the PB since it reaches even lower frequencies than the SB. Recently I´ve been listening to some contemporary organ music (Sarah Davachi, Claire M. Singer) and other music that can benefit from going under 20Hz. But it´s also a cost issue, maybe I´ll have to go for the SB.

Looks like the same scheme that the Vandersteen 2W uses.

Maybe a used one of those would work, and it comes with a crossover box to roll off (high pass) between the preamp and amp… so it only works with an integrated if there is a tape loop back or similar.