I have some radio chassis that I got without the output transformer (would have been on speaker) .,and most of these radios might put out about 3-6 watts ( push/pull 45's/47's ,push/pull 6F6/6V6 etc,so I found a 15 watt Universal output transformer that has a frequency response of 150hz-15khz (same with interstage driver/phase inverter replacements yikes!!)..... and that seems not very good at all (okay for tabletop but not big console),especially if I plan on building or using big hifi speakers and cabinet console type thing,and also increasing coupling capacitors maybe.... Would this sound bad and "middy" or nasally ....,or would the frequency response be reasonably good because the radio would only be putting out roughly half or less the output wattage of the transformer..??

Depends on the "fine print" of the transformer specs. The transformer probably has fairly flat response across the specified frequency range but starts to roll off below and above the 150 Hz/15KHz points. How much it rolls off may be spec'ed, may not be. However, you will probably get sufficiently adequate response below the 150 Hz point to make the setup sound acceptable. Truth to tell, most original audio output radio transformers probably had relatively high low-end cutoffs, since to get the low frequencies a fairly sizeable transformer core is needed. Some of the high-end AM radios "back in the day" had massive audio output transformers, but most did not. If the set that you are dealing with originally had a speaker-mounted transformer, you almost certainly won't be degrading its sound quality with this replacement transformer.

:Depends on the "fine print" of the transformer specs. The transformer probably has fairly flat response across the specified frequency range but starts to roll off below and above the 150 Hz/15KHz points. How much it rolls off may be spec'ed, may not be. However, you will probably get sufficiently adequate response below the 150 Hz point to make the setup sound acceptable. Truth to tell, most original audio output radio transformers probably had relatively high low-end cutoffs, since to get the low frequencies a fairly sizeable transformer core is needed. Some of the high-end AM radios "back in the day" had massive audio output transformers, but most did not. If the set that you are dealing with originally had a speaker-mounted transformer, you almost certainly won't be degrading its sound quality with this replacement transformer.
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ah okay. Thanks!