[RodB]

You could start by taking voltage measurements of the tube pins before then after the set warms up and volume drops. Leave the volume control at the same level for all measurements. Compare them with the chart in the schematics. That should show you what stage to look at. Tubes and resistors are heat sensitive and are generally the first suspects. Also consider that four of the tubes are loctals and need to have clean pin connections otherwise they cause strange problems. Be sure the B- common is not at chassis ground. You are fine with the 22mfd in place of the 15.