I believe the truth is somewhere in the middle regarding REL / SVS and other subwoofers.
In what concerns the price vs what you get I believe SVS is hard to beat. You get a lot for your money.
If the room is not treated with low frequency absorption (at least a few panels deeper than 15 cm) then a REL or SVS (similar sized subwoofers) will sound similar even though they might measure differently.
The REL High level connection.
This I believe is superior to SVS low level connection:
@Ingenieur explained it better than what I can.
Another thing to consider: amplifiers do not amplify the signal in perfect match between lo level RCA output and speaker output (it may be "close enough" but not perfect). For example: at 10% of volume you will have the power output at 10% and RCA output level at 10%. At 50% you will have the power output at 50% but the RCA output level at 52% or 49% or whatever ... it's not perfect.
Measurements:
This don't tell the whole story. Yes, you may be able to fine tune with DSP and have a linear frequency response but that is not actually what you can hear. You hear that response + speaker decay + room reverberations.
The speaker decay and room reverberations in turn may prevent you from not hearing other sounds within the music (like high frequency sounds) or not hear them properly.
Yes, a SVS subwoofer may get you lower frequencies (down to 15 Hz or lower) and a flatter response (when measured) but might also get you a slower response aka more speaker decay + a slight delay to main speakers output. This will generate a more pronounced bass feeling but less capacity of hearing the other sounds within music (hence audiophile try to avoid this kind of subwoofers or any subwoofer)
One example: imagine staying outside and someone is ringing a tiny bell a few meters in front of you. You can hear it clearly, you can say exactly from where the sound come etc. If a noisy truck passes a few meters behind you ... you will not hear the tiny bell any more. The bell is sending the sound wave towards you but you can not hear it while the truck passes. If you "measure" the sound you will record both the low frequencies generated by the track and the high frequency generated by the bell but you will not hear the bell.
A measurement is a good "tool" to integrate better the subwoofer, to understand it's capabilities but they don't show you the decay of the speaker and how will actually sound. That's why a wilson audio speaker will sound better than a cheap 1000 Euro speaker even though on paper they might measure very close. That's why you have tons of amplifiers with lot's of power and good THD yet some sound better than others. A digital amplifier beats a class A or AB amplifier by miles on any measurement yet it sounds worse.
Build
SVS SB-1000 Pro has a 12 Inch driver, 325 W amplifier, 20 Hz @ -3 db, weight: 11.9 Kg
REL R-328 has a 10 Inch driver, 350 W amplifier, 21 Hz @ -6 db, weight: 20.4 Kg
So the SVS get's you lower frequency, a bigger speaker so probably louder volumes yet it is much lighter --> I don't know if it's cabinet is good enough to sustain the tremendous forces that are generated because simply put: there is considerably less material used in the cabin and reinforcements.
My conclusion: Whether you go one way or the other one will need to treat the room first and the speak about what subwoofer is better. If the room is not treated then you can throw through the window both subwoofers. Both will sound boomy and will just shake your room and your ears but the music will be terrible.
Once you treat the room you will also hear the sound of each subwoofer and it's capabilities. At this point the room treatment will set you back a few thousands of dollars so paying a bit more for a "faster" subwoofer would not be such problematic any more.
Low frequency sounds are very hard to judge properly and evaluate: It's easy to hear / feel lower frequencies because they just shake you more. It's easy to hear a higher volume of bass because it will be louder. Details in bass, instruments sound ... these are harder to hear and first you need a proper room that let's you hear that.
My room is treated with about 2 cubic meters of absorption (70 cubic feet). I will add in the future another cubic meter of absorption.
Currently I have a very cheap subwoofer that I could not integrate with the main speakers before treating the room: it sounded much slower than the mains / muddy / boomy ... really bad. Now it sounds much better and I could integrate it. It still is a bad subwoofer just that it sounds 2 or 3 times better than initially did.
I plan to test in the near future the REL 328 (second hand) and SVS SB-1000 Pro (new) and get back here with my feedback.
I also have REW and UMIK 1 microphone so I can measure both.
I will keep the better sub (better for music).