Interesting how this thread has rather moved towards another topic...
I'm sure my previous posts fx "loops" can be found out there in the ether, however for those who missed them.....
The main problem with fx "loops" are that they aren't loops; they are inserts, and are usually a gain structure disaster.
In pro audio environment (say in a mixing consul) you would only use gain control processing in an insert, ie compression, limiting etc.
On a mixing desk all other effects would be added by sending signal to an aux bus and remixing this with the original signal. This is the best way of adding effects to a signal.
Ideally you want a parallel loop, with adjustable send and return levels, and run the effects return with no dry signal ie 100% effected.
It is quite possible to design a parallel effects loops that adds only one very linear triode stage to the signal path (which can be unity gain) and works very well. Unfortunately this adds significantly to the cost of the loop, which I expect is the reason a lot of "loops" aren't.
Regardless, unless you feel you need to have control of your effects, in my opinion, better results are achieved by micing the amp and adding the effects via the PA.