I think it should work, the manual shows a guitar plugging directly in, and they recommend using a preamp with a speaker sim on it, so I don't see why it wouldn't work being line level. The part that confuses me though, is the (4) 1/4" balanced inputs. Normally from what I know, unless it's servo balanced, I don't think you are supposed to use unbalanced connectors. They show a guitar going directly into the 1/4" jacks though, so dunno.

Another option is using your line out on the Spider, and then using a 1/4" to RCA adapter, so you could go directly into the "Tape In" on the keyboard amp. That input should work fine with yout line out on the spider.
"The fool doth think he is wise, but the wiseman knows himself to be a fool." - W.S.
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^ That would work perfectly. The Spiders Direct Out already a Speaker sim on it. If your gonna get that, why not just trade in the spider for a Pod?
Fender Tele > Line 6 Verbzilla > Line 6 DL4 > Line 6 Podxt (With Model Packs and FBV Shortboard) > Yorkville 300-watt Poweramp > Marshall 1960BV
Thanks for the help i wouldnt get much for my amp, they just dropped the prices to 200 dollars. And i would like to line it out at band practice, keep the keyboard amp there, and just use the amp that i have at home. Plus i think i blew the speaker on my amp. Soo