Right, we can infer that the maximum SPL is 136 dB. (It's odd that the specs don't say so directly.) But if a concert ever gets that loud, run away if you value your hearing! Anyway it seems as if you should never need to turn that pad switch on, as long as the mikes are being properly powered (since less-than-adequate powering can mean a less-than-specified maximum SPL). That's very nice.
I wasn't able to find detailed specifications on Edirol's Web site for the UA-5, but they say that its inputs are on RCA sockets. If so, it sounds as if these might be consumer grade line-level inputs rather than microphone inputs as such. The 22 kOhm input impedance goes along with that. But then I don't understand how you are powering and connecting your microphones. Is there some other piece of equipment between the microphones and the inputs of the UA-5, such as a preamp?
--best regards
P.S.: mshilarious, the series resistors in a properly made in-line pad will have no real effect on standard phantom powering; they should be well within the allowable tolerance for the 6.8 kOhm feed resistors in the first place. If not, then I wonder what value of series resistors you are using.
The other thing is that the noise contribution of a condenser microphone's capsule naturally has a 1/f characteristic. It may exceed the FET or tube noise up to a few hundred Hertz, but at midrange frequencies and higher, the FET or tube's noise predominates. Now if you look at equal loudness curves at the level of typical microphone self-noise, you see that our ears are some 30 dB less sensitive to noise at the lowest frequencies than noise in the midrange, especially the upper midrange. So in terms of auditory perception, it really is the first active device which determines the noise output of the microphone--not the capsule. Since the usable signal output decreases by the amount of the pad, but the noise level does not, the signal-to-noise ratio decreases by a corresponding amount whenever you use the pad.
You are certainly right that this loss of s/n performance may not matter in every case--and if you have no other option, a little extra hiss is certainly preferable to hard clipping. But built-in pads in microphones should be avoided if there is no real need to use them, and in any situation other than clipping the microphone's own electronics, in-line pads offer an alternative that doesn't fundamentally lower the s/n ratio.