There is alot to unpack with your query.
First of all, the Heaven 11 Billie seems to be a really cool all in one and a bargain at anywhere near its price. Its likely to be compared with other all-in-ones like the Naim Atom or NAD. Its a superbly executed amp aesthetically and at its price, it should attract alot of attention from those new to the hobby, for those looking for a compact second system or for those looking to downsize box count and cash outlay. Is this pricepoint where the law of diminishing returns kicks in for a pre/dac/phono stage/amp? Probably. It might be all many ever need...but....is it a giant killer? Well, it depends on the giant right?
There is no free lunch, no matter who we are or what we wish. You may be the only person EVER to cross shop this amp with the XA25. They are NOT comparable in hifi terms but then the Billie and Pass are in different universes when it comes to price. You need a dac, a phono stage, a preamp and the associated cables to compare them. The XA25 would likely anchor a $15-25k system whereas with the Billie you could assemble a system for less than $4k...your money, your space, your decision.
The final part of my post is to carefully tread...I enjoy Huff's reviews. He is a nice guy by all accounts but he absolutely, positively and unabashedly is not remotely monogamous when it comes to gear. He falls in love with MOST EVERYTHING he reviews and in quite a few instances, he purchases the review sample. Then, some time passes and he sells the gear on. Nothing wrong with that but there is a pattern established. For a long time, one could see a pattern where he would buy something, do the review, exalt it, upgrade, repeat and then it would seem he would sell it all and start over as if the credit card bill came due. His more recent reviews seem to be more manufacturer driven, where they send the piece, either complimentary or at industry accommodation and then the review happens. For me to take his reviews for more than entertainment value, I would want to know how long he had it in his system, how it was provided to him (free, discounted purchase or on demo to return to the manufacturer). So, what does this mean to many?
Well, I believe he provides his honest opinion when reviewing but I also don't believe you will ever hear a neutral or negative review from him (or any other reviewer for that matter). I have emailed back and forth with him before and he was super responsive and forthcoming, and I appreciated the interaction. He might be the first to tell you though that his reviews should be taken in context and that you should try something for yourself. My only caveat is his reviews do not necessarily differentiate products in an articulate manner. You might be thinking because he discussed the Billie and the XA25 within the same review that the products are worthy of comparison. That's highly unlikely. They both play music....and they both play music.
Listen, its far easier to get excited about a $2k amplifier than a $25k amplifier....because if you can be satisfied with the Billie then you have truly found the holy grail in hifi. If you can listen and be happy, then thats all that matters. I think its great that someone is spending the time reviewing gear at every price point, that is a service to the hobby and the public who consumes music. I truly envy you if the Billie checks all of your boxes and most Audiogoners wish they didnt hear vast differences between power tubes or interconnects that cost more than the Billie.Try it, it might be everything you need or want. I certainly appreciate that products like the Billie are being introduced and I hope they experience a high volume of sales. Everyone wins....but it would be a bit of an overstatement to think that the Billie and the XA25 are apples and apples.