Thanks to Steve Palmer for providing this info after his recent trip to Aachen.
THE TIVOLI STADION - ALEMANNIA AACHEN - 24,816 Capacity (3,616 Seats)
There is a free bus that goes from the station to the ground. A taxi will cost around €10 and takes between 5 and 10 minutes. If you want to walk then its over 2 miles away.
There are a couple of bars by the ground but the best place is probably the petrol station.
Get in early to get a good view otherwise you end up with squashed against the fence. Cost of match ticket is €8.
You can get to Aachen on a Rhine pass if there is more that 2 of you going.
There are not many left luggage lockers so my advice is to leave bags at Köln.