It's worth noting that the answer to this question may be very different depending on what particular headphones you have.
Some headphones are easy to drive, so they sound pretty good on almost anything, and don't benefit much from a fancy headphone amp.
Others are more difficult to drive - and the amp can make for a huge improvement.
And, of course, some headphones are just junk and don't sound good on anything.
I'm sure you can find some headphone that sounds OK with the built-in headphone output of a MacBook Pro...
What a better amp will do is to widen your choices - and make many of them sound better.
It's also worth noting that SOME little tube headphone amps are actually kind of wimpy...
And, finally, many low cost "tube" headphone amps are actually hybrid amps, with a tube stage, but a solid state output buffer.
(For example, the million different variations of the now iconic Millett Starving Student design.)
You can also find literally thousands of cheap little headphone amps, both tube and solid state, on eBay...
Some sound good... some not so good...
(And, with all the other potential differences, the particular tube they use is only actually a minor factor.)
I'm also going to take yet another opportunity to try to dispel a common myth that costs some audiophiles a lot of money - often for nothing.
It is true that some tubes, and especially certain modern ones, may be of extremely poor quality, which may lead to poor performance and a very short life expectancy.
HOWEVER, the vast majority of differences between specific tubes, which audiophiles like to discuss endlessly, are really just due to tiny differences in electrical properties.
Like different brands of light bulbs, tubes of the same number are not precisely identical...
So, even though they're interchangeable, an RCA 12AX7 has slightly electrical characteristics than a Telefunken 12AX7...
And, an RCA 12AX7 made one year, or at one particular factory, may have slightly different characteristics than another...
The important thing to realize is that, just because they're different, that doesn't mean one that or the other is BETTER...
So, for example, an RCA 12AX7 made in a certain year may sound really good to you - WHEN USED IN THE PARTICULAR MODEL OF AMP YOU OWN WITH YOUR PARTICULAR HEADPHONES.
However, the reason is NOT that it's "a great tube"; the reason is simply that the characteristics of that particular tube happen to work well with the particular equipment you have.
(So, unless you have the exact same amp, and the exact same headphones, there's not much reason to assume that the tube that sounded so good to that reviewer will sound at all good with YOUR setup.)
If you look around, you will see extravagant claims, and sometimes absurd prices, associated with particular brands and styles of tubes....
But they're often based on the fact that a certain reviewer found that they work really well in the particular gear that he or she has....
Unfortunately, once you understand how it all works, you'll realize that this doesn't mean that the tube he loves will sound better in OTHER equipment.
Back when most of those wondrous "new old stock" tubes were made, and originally sold, vacuum tubes were considered to be commodity items - like light bulbs.
While certain manufacturers worked to make their versions more tolerant of vibration, or slighty quieter, or more attractive, very few claimed a difference in how they sounded.
(Which makes spending that extra money a complete waste for anyone who doesn't have exactly the same combination of equipment and the same preferences in sound signature.)
YES, but even a CHEAP headphone amp (tube OR solid-state) beats the built-in chip of the MacBook Pro...
But would the difference be worth it is the question. It took me a while to find a good combo as I combed through lackluster amps. My pick, A-100 is $249. I would pick it over the majority of offerings in that price range. However it is bulky for a desktop amp.