The Sunn Beta amps are liked by some, some people can't live without them, especially the preamp only version. The amp uses CMOS for distortion and JFET channel switching. You'll need a footswitch to toggle between the channels or you can play through both at the same time. There is a clone footswitch or I can post a schematic (PM me). Speakers in the combos were nothing special.

Problems: There is a rare IC that manages the channel switching logic. Hard to find if you need one. I designed a little piggyback fix it that's an issue. The biggest issue is there are many 2uF non-polar electrolytic caps in the preamp. Replace them all! The voltage regulators for the preamp run hot, could use a heatsink or mount on chassis. That's the most common failure, amp won't make a sound when they go.

The old Sunn Forum is down and I don't think it's comming back. It's a shame, there was a lot of good info there.

Could the line voltage for the amp be set to 240VAC but the OP be running on 120VAC?

Unless you have a cable you can cut up, you can wrap a length of wire around the long barrel part of the plug and connect that to ground. Then simply touch the end contact of the plug to the pot terminal. You'll only get one channel but we just want to see if the power amp will pass a signal to the speaker.

Two things look wrong. The voltages on C17 and C18 are too low. This could mean that the Rectifier is bad (looks like the schematic calls it DB:) or the transformer is bad. There are other possible causes but the power amp should still make some sound.

The voltages on C2 and C6 should not be zero. this will prevent any guitar signal from getting through. Measure the resistance across C2 and C6. You might get an unstable reading, but not measure a short or low reading. Now measure the resistance across R6 and R11. Again you might get a strange reading but it should not be higher than 1000 ohms.

Can you suggest an alternate testing method?

You can use the headphone output of your computer if you have the cable to make the connection.

Your voltages suggest a problem in the preamp. Try to measure those voltages again with the preamp IC removed. Are you sure that you didn't create any shorts on the solder side of the PCB when you replaced components? Is the IC plugged in correctly? Plugging it in backwards will damage the IC, you'll need another one.

Do you have a DVM? Need DC voltages at the speaker output (Speaker disconnected), and pins 1, 4, 7 and 8 of the IC.

The problem with most transistor power amps is that the Safe Operating Area protection circuits get activated when the amp clips with a speaker load. Driving a resistor dummy load is not a problem. The Polytone has no protection circuit other than a simple fuse. Drive it hard and it will eventually toast the output transistors. Most protection circuits are too simple to allow a big enough operating area unless the instantaneous voltage across the output transistor is limited to about 50V. There are some exceptions, and MOSFETs don't have this problem. Chips like the LM3886 have a much more complex protection circuit and they perform better. There is a graph on the LM3886 data sheet that shows the SOA. It's pretty impressive. Current limiting sounds bad. Voltage limiting is good. You need to stimulate a speaker with voltage and let the current go where ever it wants.

One other thing to look out for is overdriving the input of any chip. On the chip data sheet there will be an Input Common Mode range. Don't drive the inputs beyond it or the output will not obey the input. The output usually slams the opposite rail. It sounds really bad but you won't be able to diagnose it without an oscilloscope.

That transformer is too much voltage for a TDA2040 and is too light in the current department for an LM3886. A 48V transformer will make about +/- 33VDC. At that voltage an LM3886 will make about 60W at 8 ohms. The transformer is rated at 57.6 VA. If the transformer VA is equal to the power output of the chip, it will run very hot and will probably eventually fail if you play loud. If the VA is twice the power output, it will run warm but probably survive. If it's four times to power out, it will run cool and never give you a problem. If you increase the transformer voltage above the stock value, some changes to the rest of the amp will be required to the circuits that make +/- 15V to keep them from burning up. As Roly suggested, build an external power amp.