For those of you who love classical music

Couldn't really say which particular recordings, but for checking out equipment it's hard to beat Shostakovich. The 12th Symphony is a good starting point, or maybe the 7th. Or just go straight to the second movement of the 10th.

Vaughan Williams can also be useful. The 4th is as violent as anything Russian, but violence is--perhaps surprisingly--not usually far from VW except in the purely pastoral pieces. Try the Piano Concerto, the 1st movement of the 9th or the 4th of the 8th.

You haven't said why you went off listening to classical over the last 3 years.

I listen to classical music for the most part, including a LOT of opera. These are the classical lynch pin constants from my initial listening test list.  

Puccini's Butterfly, the Scotto/Domingo recording:

from Tu, Suzuki, che sei tonto buona through Con onor muore

Rangstrom: Havet sjunger

Saint-Saens Symphony 3 Escherback / Latry SACD if you can play it.  Particularly movements 2 and 3.  Organ goes down to around 32 Hz in the 2nd half of the 2nd movement

Wagner Reingold Solti recording

Prelude & transition between scene 2 & 3

Wagner Walkure

Overture to act 3

Rach 3 first movement Ashkenazy or Kern

Debussy

La Mer Pekka Salonen

Dvorak Symphony 9 Kertesz

First movement

Donizetti:  Stuarda  Sills

Deh! Tu di un Umile preghiera through Ah! Se un giorno

Scriabin Sonata 5 - Petrov

Brahms Variations on a theme by Paganinni Angelich

Elgar Cello concerto Du Pre first movement

Crumb Vox Balaenae

(For the record the balance of the initial tracks include: Dave Carter & Tracy Grammar, Stan Rogers, McKennit, Sting, Pink Floyd, Growling Old Men, Santana, The Animals, Harpeth Rising, Rodrigo y Gabriella, Boiled in Lead, etc.)  

That said the Berlioz is a good choice I like the 4th movement when you can hear the guillotine blade fall and the head bounce down the stairs.  The 5th movement is also a good challenge to put headphones to.  Reverberation, clarity and decay of the bells, and deep and clarity with the initial statement of the dies irae.  

However that's me, and I probably do the same as everyone else, pick music I like, music I know inside out, and tracks that are challenging in their own way.  If I were to compile a list of suggested classical tracks I'd chose more main stream works. What it comes down to in the end is the ideal set of test tracks is as individual as one's library and taste in music.  

I've always wondered about that.  I play guitar, 6-string bass, and mandolin, and each has a significantly different scale length.  I haven't found it that difficult to switch between them, as you tend to have to adjust anyway since the fret gaps are narrower as you go up the neck, but I imagine it would be more difficult on a fretless instrument.  I have a violin myself, but haven't really put any time into learning it yet.