Need help placing a second subwoofer.

I use mine at the inside of my speakers very close to the same plane as the front baffle of the main speakers. You having two different brands may create a level matching issue. Also, on this arrangement, try front firing and side firing.

Since every system and room is different and your using less than four subs this method works well.
https://www.youtube.com/watch?v=AV3oLLMgS-M

If you decide to use this method and while your M&K is still at the listening position draw a map of your room walk around and note all the areas where the bass frequencies are the loudest. These are YOUR rooms modes.

Place the M&K in or near the loudest mode and the Velodyne in or near the next loudest mode. IME running L/R to each sub (ie mono) will increase the room loading.

For economical custom length long runs of cable try Blue Jeans or Monoprice Cable. 

I purchased a new (M&K) Volkswoofer from Jonas Miller Sound on Wilshire Blvd. in 1979. A friend has been using it for the past 38 years without a problem. 

Have fun with it.

M-db
I can’t believe it, but that’s where I purchased my first ’high end’ M&K system back in the 80’s. Jonas Miller Sound! They put on ’School’ by Supertramp, and when that kid screams, it scared the crap  out of me, so I bought their best system then and there! Had it till last year when I gave it to a friend.

1111art, it is fun to play around and experiment but it will never be an optimal situation. The only thing you can do here that makes sense is putting the little sub on top of the big one between the main speakers up against a wall. I am thinking like one who runs his subs up to 120 Hz.
If you are using them below 60 Hz with just a low pass filter you can put them anywhere. In corners is always best as you get an easy 6 dB increase in efficiency. Next is up against a wall for an additional 3 dB.

I personally think all subs in a system should be the same. Next would be dividing it up in pairs say two of one kind in front and two of another in back. 

Crime of the Century, Crisis What Crisis and Breakfast in America are three amazing albums by a group of amazing a musicians and all three albums are beautifully recorded. Should be in every music lovers collection. If you do not have them, get them, you won't be sorry!

Well I ended up with 4 subs, and DAMN, you guys were right. So smooth now, it's like the room and my head are one.

And yea, Supertramp, Beatles, Van Morrison, my favs back in the day.