hello people ... i am hearing some swishing in my aguilar 680 preamp. the tubes must be at least 3 to 4 years old. i haven't asked aguilar the date of the unit ... but the serial number is 107 and it's written in black sharpie! once the amp is warmed up, this swish seems less noticable ... does anybody have first-hand experience replacing tubes ... do you just plug 'em in ... none of that biasing crap, right? sorry, i know that's kinda a simple question ... but i'm really un-technical ... plug in and play ... thanks!