The i9 rear hub put chris kings "bees" noise to shame, they are very loud to me...

Yeah, they're loud. I have them on my XC bike. However, you can change the grease and they'll get super quiet.

Thanks guys for the input so far. I'm hoping to find out how different they'll be compared to the stock wheels on my allez sprint.

Belgium is a solid rim.. i9s are solid hubs... They will make for a robust rim build but I dont know what differences you are looking for.

I've read that the rims that come with the Allez Sprint don't do the bike justice, that they should be upgraded etc.
I've also heard that the HED Belgiums are nice, that the Industry 9 hubs are nice, etc.

I'm just wondering what's nicer about them? On paper they're fairly similar wheels. Will there be any noticable difference, or are the new wheels just more durable?

I just dont know if Belgium/I9 is a significant upgrade to notice.. it's more of lateral semi-step up. I dont know what hubs are on the stock wheelset... but R460 rims weigh about the same as Belgium C2.. with + might even be heavier. Tons of varibles here. I dont think you will see a night and day difference.....

if anything it will look better because DT Swiss stickers are a disaster from day one.

The noise is coming from the additional pawls which is one one the attributes that make these wheels so great. Particularly for mountain biking you can notice a difference in performance. They are awesome and beautiful . . . works of art really. The noisey pawls aren't as big of factor on the road, but these are still spectacular hubs/wheels. I went on a tour of the plant and it was really amazing what they do. High quality engineering, top machinists, technicians, wheel builders, etc.

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David
* Ironman for Life! (Blog) * IM Everyday Hero Video * Daggett Shuler Law *
Disclaimer: I have personal and professional relationships with many athletes, vendors, and organizations in the triathlon world.

Thanks for the responses everyone.

So last question.. I have (2) bikes and (3) wheelsets. Which set should I sell?

1) FLO 30's - have roughly 13,000 miles on them.
2) DT-Swiss 460's - brand new
3) Industry Nine 25i TL's - brand new

Bikes are:
2012 S-Works Venge
2019 Allez Sprint

Will I notice any downsides switching from the old FLO30's to the new DT-Swiss wheels? The main difference I see is that the FLO30's are 30mm deep vs the 24mm deep DT-Swiss.

Not sure what you should sell, but get some moderately deep aero wheels for the Venge. It's begging for them. If you haven't bought the 25i TLs yet, maybe consider a deeper rim for the I9 hub. The Allez is a great crit bike and the I9 hub may be beneficial for crit racing.