I believe there is no need to purchase matched Alps potentiometers. Though the ordinary (blue) Alps is specified with max. 3db channel mismatch, usually they are far better.
Up to now I had a couple of them and at positions used in normal hearing conditions (@1/3 to 2/3 clockwise, not at or very near the end positions) the channel mismatch is <1db, mostly even <0,5dB.
But maybe I just had luck with mine so far?
This reply might not be popular amongst those audiphiles who equate cost with quality but......
I have had really good results with Bourns 3310 series pots.
They cost about 6 UK Pounds each for a dual gang audio taper type. I've used many of them, both in business and pleasure, and have found them to be consistant. They are only quoted as +-5% accuracy (which actually means nothing), but I believe in them.
You can get them from RS and Farnell in Europe.
One point worth noting as far as channel tracking is concerned, is that the higher the system gain (the further anti-clockwise your normal listening level), the worse will be the percieved mis-match, for any pot.
So, it's worth padding with a fixed resistor in series with the top of the pot to set your maximum gain.