Passive subwoofer too loud.

It could just be that the subwoofers are simply more efficient than the satellites so you will get more output from them at the same volume setting on your preamp (?) or other volume control device.   A simple fix would be to add a pair of Rothwell attenuators in front of the subwoofer amp.  There are many listed for sale on Ebay and here is an older review:  Rothwell In-Line Attenuators by George Pappas (enjoythemusic.com).   This would be a cheap and simple fix.  A better option might be a single input/output passive preamp although these would require another pair of interconnects.  The EVS or Endler Ultimate attenuators would also work nicely but may be harder to come by.  

Check out Schiit Audio. They have a small passive volume control for about $50 that should work fine to lower the signal to the subs.

You definitely want something adjustable so you can change the sub output to your liking. If you opt for a preamp with a remote you can change it from your listening position on the fly.

Well, if it IS wired in parallel, you could try re-wiring it in series to reduce the output.

The best way to deal with this is going to be to get a separate amp with it’s own volume control though.

Thanks Gentlemen,

A note of clarification (if helpful): I do have a separate amplifier for the subwoofer. It is a typical amp not having volume controls (not an integrated amp).

As an alternative solution: I do have a decent to good Onkyo receiver that is not currently being used. It could be the subwoofer amp. Would putting the receiver inline from the preamp via a line level be a good or better solution (not in how "good" a subwoofer amp the receiver may or may not be compared to my current amplifier - probably even trade) but as a solution compared to attenuating.

Yes, that would work. If the Onkyo has a remote as most do you're in luck to change the sub level from your listening chair.

The gain/input sensitivity of amps can vary quite a bit. Could be the separate amp you are using for the sub simply has more gain than the amp being used for the speakers.  

As noted earlier in this thread, the Schiit SYS is a $49 passive preamp that you could use to adjust the volume of the sub. Simply put it inline in between the sub amp's input and the source output. 

Also, remember that subwoofers are extremely sensitive to their placement in a room.  Too close to corners or a wall/floor boundary can increase their loudness, particularly at certain frequencies.  You might try moving the sub further away from a corner or wall placement and see if that helps. And this particular experiment is completely free!  

Can always use a minidsp and measure and EQ

Do you guys know how to connect a Receiver to an amplifier so that the Receiver can be the amplifier for the subwoofer ? That would give me volume control and a remote.

I would still have the main speakers running from my other amp and preamp.

I thought that if I ran interconnects from the line out of the preamp to the tape input of the reciver, then connect the Receiver to the subwoofer with speaker cables that I would be good.  Did not work. The Receiver is so complicated so I do not know if my idea was faulty or if the Receiver was not set up right .

If it were me I would purchase the SChiit SYS or a Nobsound variable attenuator for ~$50 and place it between preamp and sub amp.  Save thr receiver for a better purpose.