Ashdown manuals don't typically provide as much technical detail as some other manufacturers, so you will either need to get a peak at a schematic or use a bit of intuition based on the limited info the manual does provide to know how the signal flows through the amp. It becomes easier with experience and exposure to information. So check out a bunch of amps and manuals as @agedhorse has suggested.
I have uploaded a Mesa WD-800 manual. It has a block diagram near the end of the manual, which is very useful for answering the type of questions you have. Scroll down till the document reader indicates page 22 up in the banner bar at the top of the page. The block diagram will show where the Effects Loop falls in the circuit path and also indicate how the switching works. Note that the Effects Send uses a switching jack, but the way it is wired no switching occurs. The Effects Return, on the other hand, bypasses the preamp when a plug is inserted. The idea here is to allow you to slave one amp to another. Take a look at the jacks and see if this makes sense. The block diagram also shows the bypass for the semi-parametric EQ section.
Based on what the Rootmaster manual says, functionality of the effects loop and EQ bypass is probably fairly similar to the WD-800. This doesn't mean the amps sound or respond to inputs the same way, only that certain aspects of their signal flows are similar.
As far as whether it is better to use the Plex in the Input or Effects Return. The answer is related to signal matching (I.E max input level versus max output level of paired devices), impedance matching, etc. A preamp is designed to take a low level signal and boost it up to a higher level that can drive the following circuitry. But not all preamps are created equal. The amount of gain preamps provide and how much level they generate varies from one design to another. To answer the question, you need the range of input levels the Rootmaster can handle at the input, and it's sensitivity rating at the Effects Return. Also you need to know the nominal output and/or max output of the Plex. Interpret nominal as an average level with peaks and dips that go higher and lower.
For the sake of learning, take a look at the WD-800 manual on page 20, as indicated in the reader's banner. The bottom of the page is labeled 16. Note the specs say that with the input pad engaged, the WD-800 instrument input can handle 6.35VRMS with the input gain at 9:00 and the EQ flat. You could use an online unit converter to see that 6.35VRMS equals ~18dBu. So the input of the WD-800 will easily handle a line level input. Hint, you will want to get a feel for the difference between line level, instrument level, and mic level signals. Some effects returns are line level and some are instrument level.
Here is the online unit calculator I used.
dB dBu dBFS dBV to volts audio conversion digital - calculator volt to dBu and dBV dB mW SPL dB decibels 0 dBFS - convert dB volt normal decibels relatioship relation explanation analog audio absolute level true rms convertor converter decibel to dbfs converter calculation online attenuation loss gain ratio reference audio engineering sound recording dBFS dBVU 0 dB audio logarithm level converter peak to peak p-p impedance voltage pro consumer audio digital analog recording level - sengpielaudio Sengpiel Berlin
Next note the Sensitivity at the Effects return is +4dBu (line level). This means you need peaks of at least +4dBu to drive the power amp to full power. So if your Plex cannot provide an output of at least +4dBu, you would want to plug it into the front of the amp instead of the Effects Return. Note the WD-800's Instrument Input sensitivity is -10dBu with the gain and master at 12:00. There is probably an assumption that the tone controls are bypassed or at 12:00 as well.
Unfortunately the Plex manual does not include specs for its output levels (see attached manual). You may be able to get the desired info by contacting GK. My intuition tells me the 1/4" output may be instrument level as the the device is intended to work with other pedals. Whether it will be able to drive your Rootmaster to full power in the Effects Return depends on if the the Effects Loop is instrument level or line level. I have also attached the Rootmaster manual so you can work through the specs and form an opinion. You will note the level for the Rootmaster's Effects Return is listed as 0dBu nominal. So this would be a line level Effects Loop
You may find that the specs indicate the Plex will work in either the Rootmaster's Instrument Input or Effects Return...in this case use your ears and the desired functionality to make the decision.
Input and output impedance is also a factor that can come into play. I am not really qualified to explain but I'll try to give you some ideas. In general if circuits are not specifically designed to work with each other, I believe you typically want the Input Impedance to be at least 10X the Output Impedance. Under this condition the impedance is said to be "bridged," and the load doesn't negatively impact the source. However, with circuits that are designed to work with each other, you may sometimes see the Input and Output Impedance are equal, as in the Rootmaster's Effects Loop. In reviewing available manuals in my collections, this is a bit unusual.
Ramifications: If the Input Impedance is too low, it can load the output of the preceding circuit down and degrade the frequency response. If the mismatch is extreme enough, I believe you could potentially damage the driving device, but this would probably be unusual unless you are pairing gear that is really not intended to work together.
which is nice
