Oooh, ooh, ooh, call on me! I know the answer to this question!!
I just went through something like 15 different overdrive and clean boost pedals in the past year looking for something "better" than the DOD EQ I've been using since the late 1980s into my RG100ES, and I ended up with three viable options that work to boost the gain in the red channel:
1) the DOD FX40 eq pedal with a tiny cut in the lows and highs and a tiny boost in the mids (gives great sustain)
2) a Tube Screamer-type pedal that I modded to keep more of the low end signal, and
3) a Tech21 Boost DLA.
Out of the two dozen pedals I tried, all three of these give the same singing sustain in my Randall SS amp. HOWEVER, if you have the gain pegged in the red channel no pedal will give you a volume boost. It just gets more saturated. The green channel, however, can be boosted quite a bit in volume. So if, like me, you like the red channel sound but just want a bit more gain/sustain, then an eq pedal is the cheapest and easiest way to do it because you have total control over what frequencies get cut or boosted. You can fatten it up OR "cut through the mix" to your heart's content and change your mind at any time by just pushing a couple faders up or down a quarter inch.
A tube screamer-type pedal CAN work, but it might suck. I tried three different ones and they sounded wildly different in my amp. Two sounded thin and nasally as heck.
And, if you're patient an haunt eBay like me, you can get a Boost DLA pedal which gives you boost and delay in one pedal with a volume control and a tone control for the delay echoes. But they are pricey new and kind of big and clunky as well. I ended up getting a nice used one for something like $60 but it took a couple weeks of searching before I found one for less than about $90.
The nice thing about the Tech21 is I can use it as just a boost OR add the delay. The nice thing about the overdrive is if I'm in the mood I can push it into crazy-gain/fuzz territory. The nice thing about the eq is it just works and it's infinitely tunable. If I had to keep just one it would be the eq pedal.
If, on the other hand, you want to go crazy with the green channel then a simple EHX LPB-1 will do wonders. But it's too noisy for the red channel in my opinion. The eq pedal will also shove the green channel into heavy territory.