The audyssey is basic version on the 2500......the same features on the 3400.....better power, pre outs and ofcourse superb sound quality........
Final call is your bro.......we have all advised ,,,,,,,if the setup is simple then 3400 should be enough....
For my room the NR series of Marantz is more than enough......yet I went for the SR series as Sound quality matters......preouts help incase the amp section blows....still it can serve as a pre and you can purchase a separate power amp then for the speakers......