Amp or DAC?

I guess since this is in Tech Talk should probably give a more technical explanation why power is irrelevant.

Music is highly dynamic in nature. Sound is logarithmic in power. What this means, to go even a tiny little bit louder like say 3dB, not much at all really, requires twice the power. Twice. To go 10dB louder, which is enough to actually feel louder, takes ten times the power.

So going from 75 watts to 150 watts, even though it sounds like a lot, is only a measly 3dB. Barely noticeable. Insignificant.

Back to music is highly dynamic in nature. What this means, the vast majority of the time is spent listening to only a watt or two. If that. Theoretically, at least, the only time you are going to hear the difference that extra 75 watts makes is the occasional transient here or there. All the rest of the time its the first watt.  

Because, see, amplifier power is not like horsepower. A car with a lot of horsepower, even when you're only using a fraction, barely crack the throttle, it just feels so much more effortless. You can enjoy high horsepower even without ever using it.

Amplifier power on the other hand, no such luck. There's little 50 watt tube amps that convey a greater sense of power than 150 watt solid state amps. And vice versa. (Probably. Just because I never heard one doesn't mean they can't exist.)

With amplifiers the first watt is so important one of the most famous and well-regarded amplifier designers uses it. First watt. Its so important the great reviewer Robert Harley once said, "If the first watt isn't any good, why would you want 200 more of them?"

And so on. We can get even more technical. We can get into the nuts and bolts of heat dissipation and size and parts complexity and quality. But, why? Harley nailed it.

If there is a difference between the 75w and 150w per channel amp can I narrow the gap or even exceed the 150w amp by using a better DAC?

You assume that 150W would sound better, but it might be the other way around, since you pay for larger power supply and output stage (including 2x larger heatsinks). At the same price level 75W would likely be the better sounding one, but when prices are different - who knows. They might be even designed in different classes of operation and 150W can be class D. Nothing wrong with it, but only you can decide what sounds better to you. 150W is 22% louder (on paper), but likely you will be OK with 75W. When 75W is equally good and you can use the difference to upgrade DAC it is a win-win situation.

@kijanki I agree that it may be the other way around and I’ve heard  it mentioned that this can be the case. I did my own comparison of with a 60w into 8 integrated amp that is stable down to 2ohms compared to a 225 into 8 amp, 425 into 4, and aside from the sound signature differences, I didn’t feel like one was controlling the speakers better than the other. 
Granted, there were other differences as the lower powered amp was class AB vs class D for the higher powered one, but both seemed to have the type of parts and engineering necessary to drive moderately difficult speakers (LS50 and the Unifi UB5) for a lesser amp.

For the integrated amp, I think most of the improvements are DAC based up the range , since they seem to share, from what I can tell, mainly the same amplification tech.

will sound different - many reasons why, too many to enumerate

power ratings are very rough guides... not informative beyond 150 more than 50 more than 10 - depends on type of amp too

re- dac ... no

Absolute power is only one of the dimensions that matter: obviously, if you are trying to drive a difficult speaker load at say 80db efficiency, a lot of power is required. Conversely if you are driving 110db horn speakers a humble 2A3 valve amp pushing out 2-3 W is plenty sufficient. Generally bigger power means bigger power transformers and consequently greater emission of EMI, the shielding in other words gets more challenging. 
Where it really gets interesting: a clean renderer and DAC, i.e. units with a very low noise floor can increase the perceived loudness. Since the amp has to amplify only clean signal rather than a lot of grunge on top, the perceived result appears louder. NB: there is no increase in power, just perception.