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On these models with a 25L6 in series with a 90V motor, you won't have any problems with using a 6AV6 or 6AT6 tube for a preamp stage. The original tube+motor adds up to 115 volts. Adding the 6AV6 will bring you up to 121 volts; which, won't be a problem. For reference, you can use the schematic for just about any portable record player that uses a preamp stage.
I see that the cartridge is a standard 1/2" mount and you won't have any issues with mounting either an Astatic 89T/mounting bracket or a Pfanstiehl P228 in that tonearm. The P228 is a stereo cartridge and you'll need to wire the left and right channels in parallel to give you a mono signal. You may also have to adjust the spring under the tonearm to get the correct tracking pressure for the cartridge that you end up using.
There are many opinions on selenium rectifiers. Many people say that if one is currently working OK; then, it should be left. Others say that such a rectifier is a bomb waiting to go off and should be replaced. The main problem they give is low output voltage (can be measured with an ohmmeter) and they can short; which, will produce a foul odor of rotten eggs.
If you choose to replace the SR with a diode, be advised that a modern silicon diode has much less voltage drop than the original SR had. To compensate for this, you'll need to up the input resistor to something in the 68 ohm range.
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