Yes, you can just fill in the missing stuff - follow the schematic - and have a reverb.  As to MAKING a reverb pan?  I sure wouldn't bother.

LEDs are just soldered in like any other part.  Remove one or both and see what you think.  You like it you leave it, you don't like it, put them back in.

FX loop is a concept, not necessary to take it literally.  Your FX loop is before some controls, I see.   But I also see the split highs and lows output jacks over on the right.  It appears using the bass alone jack is still full range, until the highs jack is used.  Don't plug into highs, then the bass jack is full range.  Added bonus you still can use the highs and lows controls to balance your tone.    Does not the power amp have an input jack?   Try the volume control there instead of in the official FX loop.    That is after all controls.   

Bass jack out to power amp in jack.

A pot in a box?  Sure.  After all, that is what a volume pedal is.  The amp has no idea what the control is housed in.  I had a pot/box on my bench for years.  You never know when you might want to turn down a signal.  250k worked well for me. 

You neglected to circle the other difference between the two models..The power transformer.  The lower power unit uses a lower voltage power supply.   And that is where the power comes from in the first place.   You want to convert 200 watts to 100 watts?  Swap out the transformer.   And half power will only be a 3db reduction in volume.  Power isn't volume.

But it occurs to me, this is a solid state amp, you have no plans to overdrive the power amp, so why not just turn down the Master volume.  Or it might be called Post volume.   That is what your FX loop control would be doing.

No, my fault.  The little photo looked like one of the tiny Gorilla amps I see.  The kind they throw in with a cheap guitar as a "starter set".

A 100 watt amp is more than that.  But I am having a hard time finding a clear photo of the rear panel.  It looks like it says 150 watts next to the 120VAC 60Hz printing   (or 240v etc printing in other lands).  That to me means the amount of power it uses from the mains, not what it sends to the speaker.

You can easily open it up and see what IC is used for the output

You can contact Pignose and ask them.

In my experience, I rarely get out schematics for small practice amps like that.  They usually have a 5-leg power amp IC, and one or two dual op amp ICs.

AC/DC rig

https://www.youtube.com/watch?v=x2bXLaQX2rw

I am not concerned over losses in the DC cable.  I am concerned a whole extra cable is needed trailing across the stage.  Electrically it can be tiny, you are right, not a lot of current.  But practically, the cable needs to be sturdy enough you can step on it, so it needs to be jacketed.

There ^^^ is my concern.  Your 9v output now needs a 20-30 foot power cable down to your pedals.   Besides, they already make little power supply pedals to throw in your floor toy collection.

Be more specific about the complaint.  DO you mean it works well sometimes and not others?  Or never really works well?  And what exactly does it do or not do that is the issue?

If it does something wrong, if you whack the top of it with your fist, will that bring it back?  At least temporarily.

Reminds me, in my shop days, when I had a speaker sitting out and wanted to test it with music.  Just a brief test.  They sound awful sitting on a table, so I would reach over and hold the speaker facing up over my trash can.  That enclosed space under the speaker gave it enough of an enclosure to see if it sounded halfway decent.

Low end from tiny speakers?  Not likely.  To make bottom, the speaker cones have to move sufficiently to make the wave fronts.  That is hard for a speaker to do with a movement of a millimeter or so.

The parts involved are also less common than they were, but Peavey made a chorus circuit for the Mace and some others, not using a BBD chip.  They made a delay line with discrete stages.

Well, I don't mean to be unkind, but one problem I see - and that is in many many threads - is people assuming that hum (the most common amp complaint there is) will be caused by the same thing.

DO you have that same "hum is less in the middle rather than the ends" on the volume control?  Are you sure your hum is 60Hz and not 120Hz?

There are a zillion sources of hum in amps.

OK, then the ripple?  You have a positive and a negative supply in there.  Are both up to about the same voltage and free of ripple?

How did you check the caps?  Just because the value is close doesn't mean it i well connected.  Sounds like DC on the speaker to me.  If so TURN IT OFF or the speaker will be destroyed.

Does the speaker cone move one direction and stay when you power up?  Take the wires off the speaker and measure for DC voltage between them.

Otherwise check the solder on the main filter caps.  Check the main power rails for ripple.