another tip, for my 5150 II, since there is no master volume, i have to turn the post gain up to get any output from the fx send, so have to turn above 1, which is very loud.
so to record near silence without an attenuator/load box, i just use a dummy cable, just any spare 1/4" and plug it into the fx return with nothing else on the other end and voila, it is the same volume as when i have the post gain at 0. then i just stick the cab in the closet or another, not that its loud theres barely anything coming from speakers, i just don't want what is leaking through the speakers to be in the same room as me when im monitoring with the impulses.
so you dont need a load box if your using a 5150 II, and i assume the 5150 too.
i have my post gain at about 4-5, can go up to 8, the higher you go the more saturated the signal is coming from the fx send it seems.
also if you do this, it is helpful to tape the end of the dummy cable, as its crazy to accidentally touch the tip of the cable, i have not done this, but at 4+ post gain, im not going to try it