Can I swap the included 6922 tube for an E88CC tube?

I currently use the HD 58X with the 7HZ 71 portable AMP/DAC. It works but I don't know how close to it's full potential. For some songs (rarely) the song sounds OK with the volume set to 40%, but usually 60% is the sweet spot (but sometimes, I need to crank it up even more, especially with metal songs). My friend who has the same headset, he has them connected directly to the motherboard. It's probably because he has a high-end mobo that the audio sounded better directly off his mobo (20% was enough to play most songs OK). As I said in the title, I want a DAC/AMP combo because I want it to take less space and usually from what I saw online, they are cheaper than separate (but if we have anything good, worthy of going split, then why not). I plan in the future to get more premium headphones like the HD 8XX/800S/820 series (or a planar magnetic of the same price range). They say that you don't see much difference (like less distortion and more power being delivered to each driver)...

Bought the PC38X in Dec 2021. Stated on the item - and still on the product page - that they have a 2 year warranty. For the last 2 weeks the balance is not working anymore, and the sound ends up in just the R side or L side of both. It especially happens when I adjust the volume knob. I tried different cables and devices, same problem. The Drop rep so far has tried to lie to me several times, saying that: 1) I should try things I already mentioned I tried (they never read my first e-mail carefully) 2) That they're not in warranty anymore (bold lie, as per the website, and they confirmed the order date is within 2 years) 3) That they're in warranty, but no stock is available, so would I accept a $20 Drop credit instead (again a bold lie, they're available and IN STOCK on the website) Why does Drop think it's ok to blatantly not honour warranty on a defective product, and why is it ok to make-up random excuses every step of the way?

...and that's a good thing.