1958 Grundig 8058 USA

[Reece]

Connect another known good speaker to the extension terminals on the radio and check if that sounds good indicating radio speaker trouble. Is the "muddled" sound on AM, FM, or both? If you play an external source through the phono input, how is the sound? The external source could be a transistor radio with cable plugged into its earphone jack and patched into the phono input. If sound is good this way would point to the audio/speaker section being OK.

Tube plates will have B+ voltage on them, varying by design of the radio to suit the tube. Your - meter probe will go to common B- and the + probe to the plate pin of each tube (look up tube basing diagrams.) The schematic will show approximate voltage at each tube plate or if not, then a typical range of voltages can be read from a tube chart. If the voltage is way high or low the circuit around the tube needs to be checked.