silly wiring question

so I just finished installing a new stereo in the Tacoma. I used a 4 channel amp, and wired front and back off the 1st 2 channels, leaving 2 channels for a future sub.
I just acquired a cheapo powered sub, and I wanted to try it just for kicks until I get my actual sub. I already have speaker line waiting for a new sub, and I don't want to run new power etc just to try the cheapo sub.
I am not sure how to bypass the wiring to just get my existing speaker line to the sub, without using the internal wiring in the closed sub box...(It's one of those cheapo plastic bass tubes.) I tried hooking up the speaker line to the green and black on the bass tube...and got nothing...?
How can I try it with just using my waiting speaker line from my good amp...?
Impossible?

 

You should be using all four channels of that amp if you have front and back speakers connected to it.

There won't be any input terminals for the subwoofer speaker on the tube, only power and signal. If you connect an amplified signal to it, you'll likely smoke something.

If you want to temporarily use the tube sub, supply power and signal to it.

 

I want to bridge ch 3&4 for the new sub...ch 1&2 supply plenty of power for the door speakers.

 

you cant use a amp for a already amplified sub

 

As long as the amp is 2-Ohm stable in stereo mode, and the 2 sets of speakers are each rated at 4-Ohm loads, you should be fine (excepting obviously no front-back fader). Amp shutting down/going into protection mode is a sign that things aren't fine, and the remedy would be a different amp and/or losing the rear speaker set.

What kind of input is expected for the cheapo powered woofer - I assume RCA cables instead of high-level speaker wire because of the questions asked? If RCA cables, you could solder an RCA tip to the end of the speaker wires coming off channels 3/4 of the amp, then turn the gain way down to drive the woofer box. The input section of an amp using RCA cables is typically around 4 volts, but the amp will be capable of putting out much more than that - why the gain will have to start at zero and VERY carefully move up.

There is a real danger you will let the smoke out on one or more devices trying to drive the woofer box in the way you request. So I wouldn't recommend doing it - especially without some electronics experience - but it can be done.