I'm not familiar with the HP8753s, and don't know what style or frequency range the YTO is, but I noticed you mentioned at the end, a "power drop" at low frequencies as the original problem.
I would guess that the problem is a cracked connection at the output SMA port. I have one with this characteristic - the capacitance of the broken joint is in series with the output, so the level is too low, and drops dramatically at lower frequency. This type has a screw-in SMA barrel, which I assume got loose and broken at the joint. I haven't tried to fix it yet.
I don't know if this helps, but if it's an old-style YTO that can be opened up, it may be possible to get at that connection for repair.
Ed
