tube preamp repair - southern NJ / Phila. area / Wilmington, DE area

Could any of you good folks recommend a good, reliable repair shop specializing in tube equipment repair/upgrading, in the greater Philadelphia, PA area?
The preamp in question belongs to a good friend who is not internet savvy. It is an old but excellent unit designed and built in Paoli, PA back in the 70s. It received a very good review by J. Gordon Holt when first introduced. I've spent many an evening enjoying classical music with his system and I must say the preamp is certainly worth getting repaired. It's a legacy piece.
The problem he is having is an occasional crackling sound from the right channel otherwise no distortion. We swapped the tubes left to right and vice versa but the problem stayed in the right channel. We suspect a capacitor as the culprit but he would really like to have the unit thoroughly gone over and upgraded by a competent technician. He is using an extra preamp of mine at this time.
Any suggestions?

Many thanks, Fred