My D-180 seems to breakup and distort (farting) too quickly these days. I opened it up (first time), and checked out the preamp tubes. I have 4 Sovtek 12AX7LPS tubes installed. Inside the chassis the preamp tubes sockets are labeled from left to right, 12AX7 12AX7 12AT7 12AX7 I do not know what the installed 12AX7LPS tubes are compared to the 12AX7. Also, will not having the 12AT7 installed in the third tube position be affecting the tone and overall output? All my tubes including the power tubes were recently installed by the previous owner. Also, I am not sure of name of the power tubes, but they have a diamond insignia with the number 23 in the middle and what looks like russian writing 6n3C-E on the other side of the tube.