The gain does not affect frequency response but can affect linearity and I have measured it many times on various subs. Not all subs or speakers are created equal. I'm not discounting the SVS for being a good sub nor do I think that Genelec is a bang for the buck by any means. Does the OP have the capability to measure and knowledge to set up his sub correctly? Audyssey and other auto EQs are a crapshoot at best. I'm talking for recording purposes where many want to get it right and don't have the means or knowledge to do a good acoustic calibration.
Although UMC-200 is a bargain for the price and has great adjustability and would be a good choice instead of his Denon. Higher end processors which are most likely out of the OP price range have even greater flexibility w/ crossover types and slopes for mismatched components as they are more likely set up by a professional. But, it appears the OP is bying these for his recordings first and HT second.
The problem with any mismatched sub and speaker is they don't play at the same level when changing the volume. I'll use a real world example. We had a customer that wanted his system calibrated using a PB13 ultra with a Tannoy Revolution speaker system. With the gain on the sub at 1/4 on the dial I could not get that system to be linear. I could level match at 75db but at 85db for the speakers the sub was hot. The sub just had more to give than the speakers. if you turn the gain on the sub much lower and the reverse happened. It was just not precise. Luckily in this system we had a QSC DSP unit and could use different settings in its gain and EQ with the remote system to change levels depending on the volume of the processor. But, this became a more complicated solution to get this room right.
Now in the average HT room this might not be a big deal to some to not have good linearity and most wouldn't go to this extent anyway. But for someone that creates mixes they want their mixes to translate well on other equipment.
Although, I like Genelec I like SVS too. In fact I had 2 SVS PC ultras in my room at one time and although I deal with both in my work. I don't have any Genelecs in my home not even for my recording area. I play keyboards for fun not professionally, and without MIDI I would stink. I have JBL monitors although I would love some Genelec 8260s. I believe in trying to find the right product for the right application, so it is not about being a Genelec fanboy or an SVS fanboy or an Emotiva fanboy or going by which has better specs.
The OP has chosen to go with Genelec for his monitors and the Genelec sub is a good match for his application. If he had chosen other monitors I wouldn't be saying go with a Genelec sub.