McIntosh MC275

Hi all. I recently purchased a used MC275 in very good condition. The original owner said he had it service not too long ago. The amp appears to work fine but the tubes are very dim. I had them tested and they are fine with about 1/2 life still on them. Not sure what to do. Do I need new tubes or is there an adjustment I need to make? If I need new tubes then could someone give me an idea what to get and where to place them on the amp (which slots)? I want quality tubes so please, ANY guidance would be appreciated. Thank you