Focker, I'm really sorry to tell you that I can confirm that I had the same results. I never really solved the issue for the limited time that I had the 661 which I sold precisely because it didn't match well either my m118 or m248. Both of the Oade preamps have unbalanced outputs, so you only have one option, which is to use the 'Line In' on the 661...but as you experienced I couldn't get a reasonable gain level on the 661 using either of my preamps. You also can't use the XLRs on the 661 either because the 661's XLRs expect a balanced signal.
Anyway, I attributed the low levels to the low gain structure of the 248, combined with the low gain range on the Line In of the 661. The 661 Line In just doesn't provide much gain range.
On the 248 side of the equation, even though the 248 has adjustable gain, I don't think there is a whole lot of gain range being controlled by those knobs, so I think the output gain on the 248 must only go up to something marginally above...say...the fixed 20db of the m118/m148. I don't know what it gets up to, but I'd be shocked if someone said that the 248 provides anything more than about 20 to 25db of gain.
Actually, you said you had the stereo cranked. Hate to say it, but if you're able to get up to -12db during the show, I would personally consider that a success because I wasn't getting anywhere near -12db.
FWIW, after selling the 661, I bought a Sony D50...that combo kicks ass and sounds fantastic together and the Line In gain range on the D50 combines perfectly well with the 248. I also got great results when I combined the 248 with an R-09HR. So, FWIW, the gain range on both the D50 and R09HR are alot better when combined with the 248...and I understand the both have nice, low noise gain stages.