I had the chance to find the Custom Studio 80 Ohm for a decent price on Ebay a few months ago. Here is my personal thoughts about how they sound:
Bass: Meaty, textured, deep and relatively thight bass, with very good extension. The bass ports at Position 1: Light and thin (almost no bass); Position 2: very lean, but flat and accurate bass; Position 3: a bit boosted, but very fun sounding and very engaging; Position 4: Big-ass boosted bass (sounds good on certain tracks, but overwhelmed on some others).
Mids: Smooth (no upper mids peak / harshness), textured, a little bit recessed, but very transparent and faithful. Not as revealing and vibrant as some other “studio headphones” (particularly with female vocals).
Highs: Definitely boosted. Sounds unreal on some tracks. Could be fatiguing on crappy mp3 files / bad mastered music or when listened at high volume level, but not as terrible as described by some DT770 users IMO (I’ve never heard any DT770 model by the way). Maybe CS have less peaked highs than DT770 (I don’t know and I didn’t found any FR graph which confirms that).
Soundstage: Very good (quite impressive for closed-back cans). Decent amount of air between instruments.
Comfort: Great, but hard clamping force (thanks to the velour pads). No hotspots at all from the headband. Good padding between the drivers and the inside of the ear cups (for some with larger ears).
Amplification: They benefit from amplification (bass gets a bit thighter IMO), but they could be driven by my smartphone (volume near the top).
As an critical-listening and precise headphones fan, I found the Custom Studio sounding a lot more “fun”, less analytical and less sharper (especially in midrange) than some of my other cans, but it’s a great addition to my collection.
For those considering K550 Mkii, they sounds like CS in position 3 in terms of bass (a little bit thighter on CS though), sharper timbre, more revealing in the midrange (especially in upper mids region), more realistic (not boosted) in the highs, but a bit grainier. K550 sounds also a tiny bit more "in your face" (like a magnifying glass) in terms of soundstage, but I personally prefer them over CS in terms of comfort (less clamping force). My ears sweat more rapidly with K550 though...
Hope this could help.