the transistors are Zenith 121-47
The modern replacement is SK3004. Mouser Electronics has it under their part number 526-NTE102A. I would think that if one of the matched push-pull pairs is out, you would have distortion when the volume is turn up. I see from the list of Zenith transistor part numbers that they also used 121-47 as a driver which could also explain weak output. I have a D7000Y and the transistors are mounted in sockets. If your R1000 is the same, try gently pulling the transitors out a wee bit, shoot some contact cleaner where the leads enter the socket holes, and push the transistors back in. Don't pull them out all the way since it's easy to get the leads back in the wrong holes. See if that helps.
Steve
I have seen the output transistors damaged by the use of AC adapters that say they are 12 Volt DC when in actuality they output around 16 Volts. The original zenith transistors have C/E breakdown voltage of 16 vdc. So if you replace them with originals make sure If using an AC adapter it does not output more than 12 VDC. Most adapters can be opened up and have a 12 regulator IC like an AN7812 added so as to protect the radio. The AN7812 has a 1 amp capability with short circuit and overvoltage protection. That being said, I think that it is very likely that C20,21; 5mfd 12vdc capacitors located in the volume control circuit are defective,thus causing your low volume symptom. Also, clean the transistor sockets with a good cleaner.