Hi,
You have frequency generator with power supplies +/-5V and +/-20V. Does all power rails can source 2.5A (From my experience I think not). The amplifier frequency response need to be from 0 to 25 MHz? (I suppose that 25mhz is typing error because it means 25 mili hertz) If I understood well, you expect to have 15V output (I suppose 15 V is max and not effective).
The schematic circuit you currently use (the quality of picture is poor that is why I can't see the circuit in collector of Q1) if the output is BNC has amplification of 1 and if the input is 8Vpp it is +/-4Vmax it is normal that you don't get 15V because +/-4vmax * 1 = +/-4Vmax. Also current shematic topology has great signal distortion.
If you place 5W10ohm resistor as current limiter it is going to burn because when the frequency is low coil impendance is aproaching its resistance (0.5ohm) so at the 15Vmax (it is 10.63Vef, (10.63V*10.63V)/10.5ohm = 10.77W at the resistor).
The frequency respons at the power level you want is too much for H amplifier topology because at that topology amplifier power rails need to track signal it is relative easy at audio amlifier where frequencies are limited to lets say 100kHz, but at 25MHz it is big problem.
First you need to know output impedance of your signal generator in order to know what amplifier input inpedance and current/voltage amplification need to be. Second you need to knoe whether you +/-20V power rails can sustain about 1.5A........
Thanks,
Petar