I got some info on Taylor Swift's guitarist from a friend who bounces between here and Nashville.
His name is Grant something-or-other.The core of his sound is Matchless or BadCat amps. Matchless Chieftains or the Bad Cat Wid Cat 40. The chieftain is like mutt between a 60s ac50 and a hiwatt, sort of. The preamp is very Hiwatt but the power amp is more voxy utilizing a pair of cathode biased EL34 tubes and GZ34 rectifier with no negative feedback. The Wildcat is awesome, I have used the Wildcat at Miner Street recording here in Philly extensively. Its got the top boost and EF86 'click' channels of the Matchless C30 amps (love my C30) stapled to a chieftain style power amp. Unlike a Matchless the master volume is not a global post-phase-inverter design, its a JCM800 type control exclusive to the EF86 'gain' channel so you can use an A/B box to switch between clean top boost and overloaded EF86 tube tones. I didn't love the master volume drive tones on this amp, but they were useable. Pretty cool for lightly driven british invasion sounds or, guess what? POP COUNTRY! The Wildcat is spankier and more articulate than the C30 or Classic Cat but still very voxy in a mid-period beatles way. The chieftain is punchy and spanky like the Wildcat but has a much more Hiwatt sound than the Wildcat to my ear. They are similar as heck though. Its an apples and apples comparison. Both are loud as hell through a pair of celestions.
Live Swift's band hasrun cab-less for more than 5 years. He runs his amps into palmer loadbox/cab sim/ DIs, then into rackmount processors. He used to use the TC G force but may have moved on to a more recent model. The stereo output from the processors go straight to front of house and he uses in-ears to monitor. He also has some pedals out front of the amps for compression, overdrive and boost that are operated by a midi switching system. I don't know what models. He probably switches up. Being a Nashville guy I would expect he uses a lot of off the shelf stuff like the Nobels overdrive, TS9, dynacomp etc and maybe Wampler's boutique effects too. Wampler is a big deal down there because of his relationship with Brent Mason and Brad Paisley. The folks in Nashville love his Ego Compressor. I believe Taylor's guitarist is a Les Paul guy but also plays a Dusenberg. Most of his tone is amp based and the pedals and processors are just to help tweak his base tone to reproduce the album sounds.
I think Katy Perry's guys are fractal axe FX endorsers, check their site. If I can get you more info from any of my aquaintances I will. No one I've chatted with lately knows anyone from Katy Perry's band. Based on concert footage I've seen it sounds like they mainly build their patches around the Marshall Plexi amp models. I could b talking out my ass though. Her band just has a really vintage Marshall sound.
As a guy who has owned plexis and JMPs before becoming a Vox fanatic I am really familiar with that 4 input Marshall sound and what simualtions of it (analog or digital) sound like. There's a crispiness in the top end and creaminess to the midrange as you drive the phase inverter and power tubes that only Marshalls, Parks & the old Laney Supergroup heads can produce and that's the #1 thing plexi style overdrive pedals and modellers try to approximate. Mater volume Marshalls don't have the same thing. They are similar if you open the master up to 10, but its not the same. Much less dynamic in how the later Marshalls crunch out. The better plexi simulations tend to carry off the harmonically complex and dynamic crispiness and the howling sustain. Tht's the key thing in the mix, the way a distorted 4 input Marshall has an evolving overdrive texture that changes as notes and chords decay. Rather than being a fat wall of grind with a buzzsaw crunch in the treble frequencies a plexi is more like an evolving synth pad. Hard to describe and I am wasting a whole paragraph waxing poetic about it.... its true of all the older british amps though the way the sound changes is different for each brand. On the other side of it an overload 60s fender has its own distinct wall of power tube grind that's more static but also very appealing. The lower gain fender preamp and phase inverter cause the amp to produce distortion in the different stages of the amp in different proportions than the higher gain, tweed style brit preamps. I could go on. You just need to collect some amps. FYI, Matchless and Badcat are 100% in the british camp tonally even though they are American made.
I don't know who Ellie Goulding is, who plays for her or anyone who knows them.
The main thing you may be noticing is that in pop music all the monitoring is done in-ear and the stage is elft uncluttered so the star can prance around like a trained poodle and summon a bunch of idiots in leotards on stage during sections with no vocal to amuse the audience. So no cabinets live. Whether these dudes run analog or digital the cabinet sound is simulated. I did one pop-country-adult-contemporary gig briefly and I took my HC30 and a marshall 4x10 and while they loved my sound for the gig the artist was trying to get me to PUT MY CAB OFF STAGE... ugh! In the studio its whatever sounds good, but live these divas just do not want you cluttering the stage or making a bunch of noise that affects the FOH mixer's ability to create an album-likre mix using PA power exclusively. No stage volume is the norm now. As in NONE. I hate it.