I have a 6 year old A/v receiver Yamaha HTR 5150. I am planning to upgrade it to probably denon avr 2805 or denon 3805. I have lot of talk on the forum about amplifier/pre amplifier. A/v receivers do the amplifing job then why do we need an amp. I would like to know under what circumstances do we use amp. I have bought pair of paradigm monitor 5s Planning on buying a center channel probably cc 370 of paradigm.

If at all if I want to buy amp to my system then I would have a budget of 500 dollars. Which would be the best one.

Can any body point me to a literature about amplifiers vs A/v receivers.

Thanks
Pandu