I'm thinking along the same lines (and looking at some of the same products) you are.
These are my thoughts, and hopefully they help you work out your decision as well. Consider these things: 1) Which headphones have you decided on, the more musical-full ones or the more reference-flat ones? 2) Do you need electric portability (battery or USB power)? 3) If you had the extra inputs, would you actually make use of them? 4) For what use you would get out of these functions, is it worth the possible, TEENY TINY compromise in sound quality? That won't make your decision for you, but it should help narrow down the field.
Another newer product is the HRT Headstreamer, although reviews and impressions are pretty scarce at the moment. Its asynch principle and digitally-controlled analog attenuator (different from a digital volume control!) are a couple of nice features that seem to emphasize an "audio purity" design orientation. But I don't like that the USB plug and phone out are on the same face plate, nor is the visual presentation that pleasing.
The Calyx Coffee has some nice control options and is stylish to have out on your desk (especially compared to the Headstreamer, which is a bit ugly). It appears somewhat bulkier than the HRT, but the HRT is TINY.
These, and the iBasso D7 (another newbie with 24/192 through USB - which I didn't think was possible in this product segment), all come from more-or-less Head-Fi-approved brands.
I might end up going with one of these, because their simplicity is elegant.
However, like you, I also value dynamics. If you are sitting in an office using full-sized cans that need significant volume, you might need to find something with a real power supply. (Your two suggestions above don't seem to be the most difficult loads, but they could benefit from a little bit of spunk!) USB-powered devices are in principle short on dynamics, given their (in-) ability to draw significant electrical current. I have a laptop with which I need to travel, and so anything DC-powered is out. Thankfully, none of these suggestions require being plugged into a wall. Your situation might be different - if your workstation is fixed, and your dac/amp is going to be fixed too, you'll have a few more options at a few more price points.
But if you travel, even only sometimes, you'll need to choose something that does not rely on mains power. That means you need something with a battery, or you need to rely on USB power, probably at the cost of dynamics. None of these 4 (iBasso, HRT, Calyx, Audioengine) has a battery.
Which is where the Fiio comes in. Not just for its battery and impressive (claimed) power output, but because the Fiio is king in terms of functionality. But how much does that matter? The only function that is actually a bonus to me is its battery. The non-USB inputs make it so much more dynamic a product, but which I could only KIND OF enjoy ONCE IN A WHILE. In particular, the 24/192 spdif or optical connection is awesome - but my laptop doesn't have spdif or optical out, so that is pretty much worthless. (I could play with it at home, which sounds fun, but the purpose of this purchase is not for home use, where I have a superior set up anyways.) The tonal controls are also awesome, but I'm assuming that the other products were developed with a very pleasant and well-rounded sound signature already. So then how much can the tone controls really be worth? With something as full-sounding as the d2000 you might want to emphasize a source that is more clarity-focused than the Fiio. With the Shures you might want something that is more musical in sound. In fact, while reviews of the Alpen are definitely positive, if you read closely, there is not too much excitement over the sound signature. I'm just guessing here, but it is likely because they put so much effort into functionality that they neglected final sound quality just a bit. In that case, the Fiio D2000 match might not be too bad. The match to the SRH940s might be a little ho-hum.
Conclusion? If you need and/or can take advantage of the Fiio's functionality - battery power, tone controls, optical or coaxial inputs - then there is absolutely no question that you should get the Alpen.
But for my situation - laptop travel, don't feel like I could make use of the other inputs and outputs - then I'd rather emphasize sound quality. Most important, perhaps, is how each of them works with your headphones, which you haven't even chosen yet!
(By the way, Headphone Addict's review of the D1 says, actually, that it has good volume, and also good synergy with his modded D7000 - might be of interest to you.)