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There is one notable factory modification that is undocumented and quite rare. This is the compressor bypass modification. Only 2 units with this modification are known to exist, (one of them was a prototype unit) but there may be more. When in this mode, there is slightly different pre-distortion EQ filtering combined with a total bypassing of the compressor stage. This results in a completely noise-free unit that responds much more like a tube amplifier. When cranked up, it has tremendous power (not to be confused with gain) that will shake the house from its foundations!
Another rare modification is accessed as an additional mode past the "mix mode". This is a compressor AND distortion bypass. In this mode, the signal goes through the 3 Band pre-EQ, but skips the compressor and gain stages. This mode allows the user to use the EQ's and effects, with no compressor or distortion.
Phil Collin of Def Leppard custom ordered an XPR with the "Metal Ace" distortion circuit.
The Superhead used on the BoSToN stage is technically a customized XPR since it has been modified with the Auto Clean circuit. Also, the Prototype Ultimatum Rack unit in Tom's studio is also basically a customized XPR.
Factory customized XPR's are of course very rare since they were usually either test pieces or custom ordered for a particular individual. They are basically one-of-a-kind units and should be priced accordingly.