a bit of a noob question, but: i'm gonna get a rather large assortment of 12AX7 tubes soon, but i dont feel like opening and closing the hoods to my Demeter and Aguilar Pre's every time i power up for a test run. can i just leave the top open as i play the pre?? or is this unsafe? btw, some of the notable tubes i'm getting are a pair of JAN-Phillips, JAN-GE and Tungsram 12AX7's, pair of JAN-GE 5751's, and a coveted pair of Mullard CV4004's. i'll post up a tone report for y'all when i'm done.