Let me see if I got this right.... You are wanting to use only the tuner section of the SX-1250 to hook up to your Spec Preamp?
If so, it is absolutely doable. Pull the preamp/main in jumpers on the 1250 out, then go from preamp out on the 1250 to an auxiliary input to the Spec preamp. You will have to use the volume control on the 1250 to get a decent gain match to the spec preamp.
Even though this will work, it will not be the best way to use a 1250. Kind of a waste really. I'd use the 1250 for another system and get you a separate tuner for the spec gear. A TX-9500 Pioneer would be my pick.