Subwoofer Question

I am a +1 on having two subwoofers.
Transformed so many different anomalies in my system. I have one up front with the LCR and then the other back behind me about 12-14 feet away.  

After using an active crossover and 4 passive woofers I built based on a design by Roger Modjeski I decided to try active subwoofers and purchased 4 KEF KC62 subs with balanced force drivers. KEF has an interesting take on setting LP and HP filters that you can read here: 

https://us.kef.com/blogs/news/understanding-lpf-and-hpf-settings?srsltid=AfmBOoofYdV0rzjlbr3BQq-70TAKqvj5oU1Qz-pdCBZRW6kppBi5c54c

Setting the LP and HP at different frequencies worked well with my box speakers, but not so much with my Quad ESL which have a resonance peak at 90 Hz, so setting HP and LP at 100 Hz was ideal. As the KEF article states with a 4th order filter the highest attenuation of the signal will be -24 dB at one octave above for the LP and one octave below for the HP. So for 60 Hz that will be either 120 Hz and 30 Hz.

So a second sub eliminates peaks and nulls or just decreases the chances of a listener sitting in one? 

Watch this.

https://www.youtube.com/watch?v=RE6g2IawzSQ

Read these.

https://www.harman.com/documents/multsubs_0.pdf

https://www.acousticsciences.com/wp-content/uploads/2024/11/Soundoctor-Barry-Ober-sub-integration.pdf

@maprik, to directly answer your question:  Could having just one subwoofer (REL T/7x)with the crossover set around 60hz pull my image to the side that the sub is on?

 Rereading my long post I realised that I did not specifically address this concern of yours so I apologise for that. If the sub has been properly set up regardless of where it is positioned it will not skew the image, the variable phase adjustment/timing and smoothing of the response will deal with this and this is why I try and impress upon everybody the importance of being able to measure. It's not expensive nor difficult to do and there are many youtube tutorials and articles on how to proceed. The best way forward is to read articles from the likes of Toole, Welti and Geddes. Dr. Earl Geddes actually wrote his thesis on this very topic and I suggest you take heed of their teachings. The posts in this thread are I'm sure all well intended but for someone new to acoustics it would appear confusing and daunting. It's not. It's thoroughly investigated science and proven in practice and all within your reach. 

The cost of the mic. which is about the same as a meal for two will be the most valuable cost effective purchase anyone can get and is IMO the only way to dial them in. The measurement system does a lot more than help position the subs it will provide a waterfall plot of the time it takes for the  sound to decay by a certain amount. If your room has a lot of hard surfaces and bare windows the sound from the seakers will take a long time to deay creating a confused smeared image with loss of detail.

This is scientifically based and not subjective in behaviour. It is difficult to conceptualise the damage strong early reflections cause which can be dealt with by placing some broad-band absorbers in the right place. Again measurement is necessary where the decay is computed for you by the amazing App REW which will provide the time it takes for the sound to decay by 60dB across the spectrum known as RT60 usually about 300ms for domestic listening rooms. RT60 depends on the intended use and volume of the space. Tables can be found online and the times will differ depending on whether it's for a lecture room, school, church etc. 

Getting a smooth frequency and decay response is paramount and I state with confidence that a modest system dialed in as per the above will get you a more immersive and rewarding experience than a big $$$ system not dialed in. All for the cost of a mic. and if necessary which it usually is some absorption.

Hope this was helpful and good luck.

If you place sub 1/7th of each dimension into the room and place near the listening position on an isolation platform, you'll be within the wave launch of the low frequencies and the direction of those notes will be perceived as coming from the mains sprakers that are producing the harmonics that are directional. The 1/7 puts the sub in the spot that's least suseptable to room harmonicas at low frequencies.