I put her back together tonight and powered her up without any issues, so I may just leave well enough alone.  The only thing I noticed was a fairly prominent hum when the volumes were all the way off.  Not sure what that's from, but I could probably live with it.  The amp has really nice tone.  She still has all the original transistors and caps, which all seem in fair shape.  I still think it would be safe to replace the 9158s with a pair of MJ15015s, as they are not germanium.  I plan to leave the 3155 alone.  Still, I've not heard that transistor issues cause hum.  I've heard that it's usually the capacitors getting old. 

Thanks all for the detailed and thoughful replies.  I am doing a few improvements to the head because I've had it in storage for several years and my son wants to start playing through it.  The head seems fine, but I want to put a grounded plug on it and I'd like to swap out the speaker wires exiting the head with a 1/4 pin plug.  As long as I had it apart, I thought I might replace pluggable parts such as the transistors and the large capacitors.  However, I may leave the germanium 3155 transistor alone and just replace the 9158s with a matching pair of MJ15015s

In any event, I really appreciate the comments about the 3155.  I would never have guessed that it was a germanium PNP.  I assumed germanium would have been too expensive for a Silvertone.  As you can probably tell, I have basic electronics skills and don't know solid state amps well.  I didn't realize that the replacement transistor simply needs to exceed the specs of the original.  Very important to know.  I always found the old Silvertone to deliver way below its stated output.  I have a 50 watt Laney tube amp from the early 80s that drives the 6 Jensens in the Silvertone cab way harder than the solid state 1465 head.     

Here are images of the relevant part of the schematic as well as the two 9158 power transistors and the 3155 lower power transistor.

I have a vintage 1967 Silvertone 1465 head and cabinet in nearly mint condition with all 6 original speakers.  It is stated to be a 150 watt amp with two channels of 75 watts each.  I've had it since about 1982 and I'm doing some work on the head.  I have the schematic and owners manual, which indicates that there are two main output power transistors mounted on heatsinks and one lower power transistor mounted on the main board.  The transistors are identified using Sears part numbers, and there is no reliable cross reference.  I'm wondering if anyone has replaced the power transistors in a Silvertone 1465 before.  The two higher power transistors on the heat sinks are labeled 9158.  From reading other online discussions, I've deducted that these can be replaced by NPN MJ15015 transistors.  However, I'm not sure what the single lower power transistor (labeled 3155) should be replaced with.  I've seen smaller Sears amps that use 3155s as their main output power transistors.  I wonder if the 3155 is equivalent to a 2N3055 NPN transistor.  I'd welcome any suggestions or commentary.