If memory serves me correctly Decibels is not in Forum; and they deal with wharfedale, yamaha etcHi, During my visit to Bangalore two weeks back I had seen a shop on the first floor (I think the shop name is "Decibel"), this is other than the Sonodyne outlet in the Forum Mall.
You are right, the shop I am mentioning is "Cinebels" which is in Garuda Mall & not "Decibels". The contact person is Mr. Jagadish C (Tel. No. 080-32935266 / 56641178).
The only AV shop in Forum Mall is Sonodyne brand shop and they dont do Marantz. But here are the ones that do:
1. Absolute Phase; Prithvi - Cubbon road, near Samarkandh
2. Audio Planet; Palani - Behind BDA complex Koramangala
3. Reference Point; Venkat - 12th cross, CMH road, Indiranagar