This is the underside of the SVT-CL power amp chassis.  The foil side of the main PC board is nicely exposed.

To access the component side of the circuit board, a few screws need to be removed on the top side of the chassis.

Here you see 3 of those screws.  They hold down the power tube assembly.

In all, there are 8 black machine screws with lock washers.

With the screws out, you can probe the component side of the main board and the underside of the power tubes.

The 16-pin cable between the main board and the tube board is short.

If you raise the tube assembly too much, the cable will disconnect.

To troubleshoot a powered-up SVT-CL, you have to reconnect the chassis, as shown here.

A convenient way to access the whole amp is to stand the two chassis on end.

Then you can probe both sides of the power amp board as well as the 6550 power tube sockets.