Looks for Headphone Tube Amp

should I use both a DAC and an amp?

Absolutely — your headphones are plenty good enough to reward you significantly by feeding it a better signal, and the DACs in iPhones and iMacs suck and will greatly limit your sound quality. At your budget I’d avoid tubes and go with a combo amp/DAC like an AudioQuest Dragonfly Red or something from Topping, etc. And get Qobuz instead of Tidal if you can — it’s cheaper and better with none of the intrusive MQA crap to deal with. BTW, you’ll also need an Apple Lightening to USB 3 cable (about $30) to be able to bypass the device’s internal DAC.  You won’t hear much difference with crap MP3 tracks, but CD resolution and up will sound much better.  Hope this helps, and best of luck.

@soix also, totally fine shelling out more for a desktop amp, and dac/amp combo. Just piecing it together over the next month.

In that case, any amp suggestions? Or amps that would pair well w my dragonfly or the portable amp/dac I just posted?

My experience is portable are generally not nearly as good as desktop, I have tried a few.  I definitely include my own A&K Ultimate SP1000 (~ $3.5K) with attached extra amp. It is good, but not nearly as good as a similar desktop equipment.

Looking closer at your headphones I’m not sure the iFi would be the best match sonically nor would a tube amp given the 99’s low input impedance.  Personally I’d be looking to a more neutrally-voiced amp/DAC like the AQ Dragonfly Red that I own so know its sonic signature well, and the Stereophile reviewer who reviewed your ‘phones also used the Red and it seemed to work well with them so you might want to read the review.  What might be an even better option for another $100 if you don’t need portability would be the Topping E30 DAC and L30 amp separates.  Anyway, just some further thoughts FWIW.