Ad30vt
The Ad30vt is a super sophisticated amp. The V Jr is a simple design. The Vox probably uses several types of current and has some really sensitive components in it. Plus it is probably highly computerized itself. Something in the Vox is probably picking up the digital signal from the computer and you hear it bleeding thru the speaker at very low volume. Plus, my RP350 runs the signal both ways so you can hear the amp play back the recording instead of the computer's speakers, this is selectable in the program. It uses the RP350 as the soundcard or something. Therefore, when you have your usb from the RP 350 connected to the computer and obviously a cable connected from the guitar to the amp, you could be getting the audio signal coming back thru the RP350 which is powered up, and going thru the guitar cable back to the amp and that signal is bleeding back to the speaker in that direction, even though the amp is turned off. Some circuit in the amp is probably working something like a speaker wire on a stereo and the signal is going from the RP 350 thru the guitar and into the input for the guitar and once inside the amp there are all kinds of sophisticated things present inside that amp. Something is probably conducting the signal from the RP350, working as the soundcard because it's hooked up to the computer's usb jack, back to the speaker at a very low bleed thru level.
If you disconnect the usb cable from the computer I'm sure that you will not get any of the computer audio coming thru the amp, most probably.
Those Vox chromies are really sophisticated amps.
Duffy
I could be really incorrect on this, but I know my PA system plays back my recordings instead of my computer and I have my RP 350 plugged into the instrument input on my PA. Except my PA plays it back sounding great just like it is supposed to. The signal is flowing on playback, back to the RP 350 thru the usb cable and out to the PA thru the instrument cable that I use to monitor what I'm recording in real time thru the Pa while I play the guitar and record. The signal goes both ways thru the RP 350. There is a software setting that you an choose for designating the RP 350 as the soundcard for recording and playback.
I was really impressed to hear it play back thru the PA even though I didn't have a separate circuit going into the PA for playback, just the guitar out to the mixing board so I could hear myself thru the PA while recording instead of thru headphones or the computers speakers. It sounded great both during the monitoring during recording and on playback going thru the PA. The RP 350 and its software is designed to work that way. Actually really cool thing.
Why yours still does it with the amp powered off is mysterious but probably the signal is passively flowing thru the amp back to the speaker. When the amp is turned on does it play the recorded playback nice and loud?
You can get it to by assigning the RP 350 as the playback device.
oke: 
