Have one of the custom builders build a variable bass boost M^3 with a line out and an input selector switch. Those wouldn't add much to the cost.
You'd end up with a nice match for your HD-600s, all the bass you could want and the inputs you want. I don't know of anything else that would fit your requirements.
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UNCLE ERIK
Vinyl, Tubes & Grado
Orbe SE -> SME IV -> Fi Yph -> Zana Deux/Si2A3
RS-1, HP-2, HF-1, K-1000, K-701, K-501, K-340, K-240DF, HD-650, HD-600, HD-414, DT48, DT880, MDR-SA5000, ATH-6, Aperio Alpha
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