For me music is purely enjoyment, so I don't know much about headphone needs for professional use (recording, mixing, monitoring etc.)
I have owned and loved my open-back Sennheiser HD580 Precision, the predecessors of the HD600, for more than 20 years. Along with the HD600, the HD580 are known to be one of the most neutral sounding headphones. A few years ago the need for a closed-back alternative arose and I have since been on a quest to find a closed-back headphone that sounds as close to my HD580 as possible.
The first closed-back I bought on that quest was the Focal Spirit Professional. Not as good sounding as my HD580, but sound was fully acceptable to me. However, the very noticeable clamping power combined with the semi-on-ear design, left me reluctant to put them on. And I have a small head and small ears!
I sold the Focals and got a pair of NAD VISO HP50. I couldn't try them before buying them, but the deal was so good I figured I'd be able to sell them without any monetary loss. I preferred the sound of NAD VISO HP50 over the Focal Spriit Professional. The sound was clearer and I would notice more details in the music. However, I could not get a good seal under my ears (even if the head band was fully retracted), unless I was lying down. Without a proper seal, the sound would suffer greatly. As mentioned earlier, I have a small head and small ears. Even so, I felt that ideally, the ear-cups should have been a bit larger too - for best comfort. I sold the NAD VISO HP50 (same price as I bought them for).
In the past few weeks, I've spent many hours listening to, and directly comparing, four different closed-back headphones: Audio Technica MSR7, Bose QC35, Shure SRH940 and Shure SRH1540. The MSR7 and the QC35 were out of consideration pretty quickly and the two Shures remained. I found that putting the 1540 ear pads on the 940 made it not only much more comfortable to wear, but also more comfortable to listen to. It added a little more space to the soundstage and made them slightly smoother sounding. Soundwise, this is the closest I've ever been to the goal of finding a closed-back headphone with a sound signature as close as possible to my beloved HD580. It was a sweet sounding combination! I'd definitely recommend anyone owning the SRH940 to buy a pair of the 1540 replacement ear pads!
To my own surprise, I ended up choosing the SRH1540 over the SRH940, even if the SRH940 with 1540 ear pads came closer to what I thought was the goal of my quest, than did the SHR1540. It was not an easy decision, as I definitely preferred one over the other on some recordings, while on other recordings it would be the opposite. One important factor was the well-known breaking headband issue of the SRH940. It is beyond me how Shure could use such a cheap piece of plastic head band (devoid of any metal reinforcement) on a professional headphone! What were they thinking? After holding it in my hands, I was not at all surprised about all the reports of it breaking! I was considering, and looking into, how one could replace the headband of the 940 with something more durable, and perhaps even more comfortable, because the sound might just be worth the required extra effort, time and money - it would still cost less than a pair of SRH1540.
Comparing the sound signatures of the two Shures here's what I found: The 940 is, as described elsewhere, the more neutral sounding headphone. In most recordings, I preferred the bass in the 940 - it was there, tight and controlled. I found the bass of the 1540 to be less well-defined and over-powering at times, depending on the bass content of the recording (I seem to be sensitive to bass, because when I think it is right, a lot of people seem to think it is too light). When it came to the all-important mids, it would also be very much depending on the recording which one I preferred. In some recordings the mids of the two would be almost identical, but in most recordings the 940 would have more presence while the 1540 would be more (sometimes just a little more) laid back. I think this is a contributing factor to the 1540 sounding more spacious to me than the 940. This came as surprise to me - that there could actually be an advantage to slightly laid back mids. I'd never experienced that before. In the treble region, I almost always preferred the 1540 over the 940. Here I should add that my ears are not what they used to be, and my hearing starts to roll off drastically around 10kHz. I can hear up to maybe 15kHz, but I need to crank up the volume to do so. To me, the treble of the 1540 is just more elegant, smooth and silky. While the treble of the 940 sometimes would show similar characteristics, sometimes it would be a bit harsher - just a wee bit - not uncomfortably so.
As a whole, most of the time, the sum of the parts added up to me thinking the SRH1540 had the more refined sound signature of the two. The sound of the SRH940 was a slightly more rough, edgy and analytical, while it was a bit more polished and coherent in the SRH1540. I am talking comparative nuances here. Generally speaking, I wouldn't put those labels on the SRH940. As indicated earlier, I was surprised to find myself drawn more and more to the SRH1540. In fact, my first impressions were those of a recessed, boring and soulless sound signature with way too much bass. What can I say? The more I listened, the more it grew on me. When it came to parting with hard-earned cash for one of these two, having an interesting alternative to the neutral sound signature of my HD580 also played a role in my decision making. As did the superior wearing comfort and solid construction that won't break (unlike the SRH940). I will probably buy a set of 940 replacement velour pads. When you put those on the 1540, the sound signature gets closer to that of the 940 (still not the same, of course) and having the flexibility of being able to change the sound signature towards more neutral could come in handy.
From what I've read is important in a headphone for mixing, it seems to me the Shure SRH940, with its more neutral sound signature, would be better suited than the SRH1540 (as I started out by saying, I don't know much about this kind of professional use). Also, the SRH1540 may hide or diminish details you'd want to hear when mixing, as it is perhaps more forgiving. The SRH940 on the other hand, tells it more like it is. It may not sound as elegant, but it won't hide things from you, and in that sense, it is more honest. For when you just want to enjoy music, you could put a pair of 1540 ear pads on the 940 (see above). Regarding the head band breaking on the 940, you could use them as is until they break, and then replace the headband with something more durable (I've seen examples here on head-fi with a bayerdynamics replacement head band - I've also seen someone using a Sony head band). Look at it this way: the total cost would probably still be less than a pair of Shure SRH1540 (assuming you are up to the job yourself) and you would have the better suited headphone for your professional mixing needs.
I note that you have been using a pair of Fidelios. I haven't heard all the different Fidelios, far from it, but the ones I have heard, have been too bass heavy for my taste. My main quibble about the Shure SRH1540 is its, at times, exaggerated bass. For you, that same bass might even be desirable? If you have a preference for bass north of neutral, the SRH1540 may be preferable to you. But if you use it for mixing, you risk ending up with mixes that sound bass light in any neutral sounding setup, no?
That can tell you these are quite fine for mixing.