The Vishay metal films will be an improvement. To really get what they are about, you will have to replace all the resistors in the signal path and all those that are in the cathode and bias circuits as well.
Coupling caps are another area you can make audible improvements, and they are usually more audible than resistors. The best sounding coupling caps are the Mundorf Silver/Gold and the V-Cap Teflons; they are also some of the most expensive.
Filter capacitors make a difference too, although less profound. Many of the Chinese filter caps have poor performance, but before assuming yours are in that lot you will want to investigate first.
Power rectifiers make a big difference too. Tube rectifiers are the lowest noise, but sag under load. HEXFRED rectifiers are some of the best semiconductor performers and are also relatively inexpensive.
Sockets and wire make a difference too. The best wires will be pure oxygen-free copper or silver. Polyethelene is the best insulator for copper. The same or Teflon are the two best for silver. Both wires sound the same but silver breaks in a lot faster so it seems to sound better at first. It also has greater corrosion immunity over the years.
Surprisingly, a lot of tube amps use silver contacts in their sockets, but silver offers lower contact performance in this case as opposed to tin-plated. Tin is much harder and is resistant to corrosion (and for this reason was used a lot more before stainless alloys were developed). It is also the same material that is on many tube pins. Over years of use, tin-plated contacts will hold up better.
Obviously if you were to do all this you would be completely rebuilding the amp. If you wonder at why some of the Chinese imports are cheaper than the domestic stuff, the materials cost/quality is one, but certainly not all, of the reasons.
