Would you please help me to think how to choose an approximate transformer with the least losses for my application? No exact transformer available at market.
Is it best, say, taking a 2600/8 ohms or a 600/4 ohms transformer?
Whatever made you think this can drive speakers , was incorrect. This tiny tube circuit can only buffer 47k Ohm phonograph signals to a pre-amp and add some hum.
A bit strange application but your 2 kW @ 600 ohm amplifier needs to put out (P=U^2/Z) 1095 Vrms to supply 2 kW and your speaker needs 14.1 Vrms to produce 25 W.
As such, you can wind a 1095:14.1 transformer with at least 25 VA rating for the given frequency (audio frequency may heat up the core more than 50/60 Hz, hence margin for given frequency) to match your amplifier to your load. Please keep in mind that 1095 Vrms is more than most magnet wires are made for, so you may need to provide additional isolation.