[morzh]

Tom,

You are right to use 10uF 450V. 12uF is also fine, but for he very first cap after the rectifier you do not want to go much higher as this increases the voltage, and your tubes might run hotter.
300V Working voltage rating is not enough. 450V is good, and 500V is also popular.
For the second cap after the inductance (whether a choke or the Field coil of the speaker used as a filter choke) you could increase the capacitance if so desired, within reason; I often go with 22uF 450V/500V.

Now, this said, I would recommend NOT to buy caps from Amazon, with names like Allecin etc.
Also for that first capacitor, my recommendation is to go with those rated as "High ripple current" or "Lowe ESR" (which is almost the same).
The temp rating of 85C is OK, if it is 100uF, it is even better.

Panasonic is a very good source, sold in Mouser and other reputable places.

If you decide to go with film capacitors, this is fine too; if you decide to restuff the existing electrolytics with new caps inside them, the new electrolytics fit better inside due to smaller size. Many people do prefer the film caps, as they do not have limited life span, as the electrolytics do; however a well chosen electrolytic will outlive us all.