I think Rush's subs are all under contract.
The one they use the most is Roger Hedgecock (here in San Diego) who works for another Clear Channel station (KOGO-600). They tried to get Roger to go national for years, but he refused to leave San Diego. Now, he has received a huge contract with Clear Channel .. and subs for Rush - which gives him national exposure. It was a smart move on Roger's part.
I believe the other subs are also a part of Clear Channel. I never listen to them .. I'd rather go home and listen to re-runs of Rush on my computer. I bought an MP3 but I haven't set it up to download Rush's programs yet.
If you do find out Roberts is on CSPAN - please ping me - but realize I'm gone during the day - working.
Walter Williams makes a pretty good guest host, too.
BookTV.org is only on CSpan2 from Saturday morning - Monday morning. I've made a note to ping you if I'm lucky enough to catch Andrew Roberts on again.