No guitar sound output - Roland Boss DR 880 Dr Rhythm Drum Machine

I am no expert about such things but the electric guitar pickups work with steel strings, wouldn't work if the strings weren't steel - the pickup is a coil of wire and the vibration of the steel string generates an output in the coil the amplifier can work with.

The pickups of the acoustic guitar are presumably glorified microphones that would usually work in a similar fashion to the above, by generating an output but if they are active devices they will need a power supply in order to generate an output, something a standard amplifier input will not provide.

An active device either needs an independent power supply to make the device work like a radio needs a battery or the device will create the output by varying the current flow of the voltage applied across it.

An acoustic guitar, unless it is an electric type, will need to be played near a microphone in order to provide a signal for the sound system and the microphone or the guitar will have to be connected to a compatible amplifier input.

Impedance matching is not so important unless high quality is required but matching the sensitivity or gain of the input to the output of the source is important for optimum performance.
A moving coil microphone has an output of just a few millivolts and a crystal microphone can produce an output of around one volt.

here's a link to the reset info-
DIGITECH RP 7 VALVE User Manual Harmony Interval Charts

Distortion is not produced in the guitar. Distortion is generated in the amp by circuitry that increses the gain until the waveforms "flat top". This creates higher harmonics and a buzzy sound. If you are getting a very low output from your guitar it is possible that you don't have enough output to get the distortion in the amp to work. Other settings in the guitar add harmonics by mixing more than one pickup. This is not ditortion but adding these harmonics changes the sound immensly.
LOOK AT THIS SITE AND FIND A SCHEMATIC THAT MATCHES THE RESOURCES OF YOUR GUITAR... You might be able to figure where the wire goes... It may be a ground.

http://www.guitarelectronics.com/c=D65u46R1H9tko6CKpPLsjBhiJ/category/wiring_resources_guitar_wiring_diagrams/

If the guitar is active and the battery is dead you will not get any sound. If the output jack or some of the controlls have been twisted and may have broken the connections on the inside.

Make sure you have the input and output cables to the correct side. The power should come on when you plug in the INPUT cable from teh guitar. A mic MAY have insufficient output to drive this. You may need to use a guitar that has a preamp built in.

Make sure you have the correct power adapter, not a substitute. Make sure your guitar is outputing enough... You LIKELY will need a guitar that has a preamp in the guitar, (not a passive pickup guitar). This device is nothing more than a small amp that drives a speaker which the sound travels up the hose. If your guitar creates a buzzing noise, then this device will do that too... Plug a set of headphones directely into your guitar to verify the sound is clean... NOTE, only one side of headphone will have guitar in it if the headphones are stereo type.

It sounds like you have NOT connected the power adapter to the box to power it. The guitar sound won't be too loud as you run the hose to your mouth and pick up the sound with a microphone such as a Shure SM57 which then goes to your amplifier. Without the power adapter the internal amplifier will not produce any output.

IF YOUR GUITAR has a built in tuner, make sure the tuner is off as it disables output while tuning... The other thing is to check if you have a volume control on the guitar that might be set too low.. Make sure if you are using 1/4 inch plug guitar cable that it is NOT a TRS balanced cable, as that will turn off the power in the guitar.

Read your manuals for both pieces of gear carefully first. You only have one amp so you won't be able to take advantage of all the FX on the Digitech as a lot of them are designed for stereo. The Digitech manual shows guitar into the Digitech and output from the Digitech into input of any guitar amp. Since you have a Marshall and can already get some great sounds by running your guitar straight into the Marshalls input, you would probably be better off using your Digitech in the FX loop. Run a guitar cord out of the send on the loop into the input of the Digitech and a guitar cord out of the left / mono output jack on the Digitech into the effects return jack on the Marshall. Start out with the effects level on the Marshall all the way down. Do the same with the Digitech with both the input and output levels. (Read the manual carefully) I think the choice of the Digitech isn't that great because it is already trying to reproduce a lot of what you can already get with just your Marshall but with a little experimentation and some careful adjustments you should be able to add to the sound of the Marshall. Hope this helps.