pioneer deh 3300ub wiring sub/4 spkrs

I am updating a system (replaced the stereo deck, upgraded the four 6.5" speakers) in a boat, and have not been able to get my subwoofer to work properly.

The previous setup had 1 subwoofer and 4 speakers powered off one 4-channel amp (Memphis Audio model PR4400).  The new setup has an RCA cable from the rear output to a new amp (2-channel Rockford Fosgate, unsure of specs) powering the sub (bridged).  The 4 speakers (one per channel)are powered by the Memphis amp from the deck with RCA cables.  The only other output from the deck is via the wiring harness (8 wires for the 4 speakers: white +/-, gray +/-, etc.), so I bought 2 boxes -- don't recall the specific term for these -- to convert the wiring harness to RCA which then could connect to the RCA inputs on the Memphis amp (all are Low Input).

The 4 speakers work great, but the sub barely produces any sound (when isolated, the volume has to be cranked to near-full volume on the deck for it to be making the expected sound of a much lower volume).  I have fiddled with the settings on the amp and the deck, but to no avail.

When performing troubleshooting, I connected the rear output for the subwoofer to the Memphis amp, to see if the issue may be amp-related with the Rockford Fosgate.  I disconnected the 2 speakers attached to channels 3 and 4 on the Memphis amp, bridged the subwoofer to these channels and plugged in the RCAs from the rear output to the RCA inputs for these channels, 3 and 4.  This also did not work and produced the same, minimal at best, volume I got with the Rockford Fosgate amp.

Then, I unplugged the rear output RCA cables from the amp and plugged in the RCA cables that were initially in channels 3 and 4 (powering two of the speakers).  The subwoofer was still bridged to these channels and suddenly the subwoofer started booming (at the expected volume, and in proper synce with the music, not pulsating randomly).  So, I was getting a solid bass sound/volume originating from the wiring harness (the 8 +/- wires) on the back of the deck.

So, there is no way to hook up one subwoofer and 4 speakers to this deck?  Per the owner's manual, it should be able to support it.

YES, hook the 4 speakers to the deck power, and use the rca's to run your sub.  OR use 2 line out converters on the front and rear speaker outputs and the built in RCA out to the sub amp.

My original wiring I used 2 line out converters to convert the 8 wires (from the wiring harness) to RCAs for the 4 speakers, which go to the 4-channel amp that powers the four speakers.  I then had another RCA cable going from the RCA preout on the back of the deck to a separate amp that was to power the sub.  In this schematic, I got good (which I expected) sound from the four speakers, but barely any volume from the sub.  If I isolated the sub in this schematic, I would have to turn the volume on the deck way up (level 53, normal-to-loud volume varies between 10-25) to get a decent sound from the sub.  I have read through the owner's manual and tried the different settings on the deck to get the RCA preout set to subwoofer (I set it up the way the manual instructed for the sub preout, as well as trying other settings when that didn't work).

Is your first suggestion that I run the speakers directly from the deck (from the 8 speaker wires in the wiring harness), bypassing an external amp completely, and only powering the sub, via RCAs from the preout, with the external amp?

Also, is it possible that I could add something to the original wiring schematic I noted in the first paragraph that would make everything work properly?  I actually called the Pioneer helpline, and they suggested that more voltage may need to pass from the RCA preout to the sub amp.  I didn't understand why, since it is an RCA preout (I would think it would be as simple as using RCA cables like I have).

I will try this and see if it works.  In my original set up, I had the RCA preout set to SUB, as you noted, but I had the amp switch on LPF, since I thought it should be since it is a sub (low pass).  I will try it on HPF instead, as well as follow your other suggestions. Thank you -- I will let you know if it works!  (Look for a response in about 2 weeks, as I am going on vacation with the boat and won't be posting again till I get back).