Where are you picking the signal up?

It sounds like your picking it up from part of the osc drive circuitry where any capacitance will have a major impact on the osc. I haven’t seen a schematic for the empeg so I can't really comment and you obviously have electronics experience or you would not have gotten so far. So I guess you know to take the signal from a driver or fan-out gate.

If this is not the problem you may just be loading or deforming the signal too much. At 11Mhz you want to keep the lead as short as possible and personally I would not shield it. It's not like your trying to pass CE or FCC.....