TKenrick

Welcome, Steve! (I'm an ex-Worcestershire dweller, born in Evesham, seems we're all coming out of the woodwork now...)

Another vote for the classical posture here 👍

I've had similarly disappointing experiences at the Bass Gallery, and at Charlie Chandler's too (pickups wired in the wrong way round and the bass came back covered in sweaty finger marks after the teen behind the counter told me that it 'played really nicely'...). I use Andrew at Richmond Guitar Workshop; he's done cavity shielding, pickup and preamp installs and setups on various basses and the work has always been top notch (he used to work at Charlie Chandler's in Kew before it closed).

A couple of charts that were requested by students: Big Mountain - 'Baby, I Love Your Way' Roy Orbison - 'Oh, Pretty Woman'

Voice leading is hugely important for bass players to understand, because it's one of the main things that separates good players from great players; if you can find the path of least resistance through a chord progression using inversions then all sorts of interesting things can happen in your basslines. This is great to hear. There's a certain mindset out there that makes people think that learning theory* will destroy all creativity, but I think the reverse is true; I'm endlessly fascinated by pulling apart the music that I love to find out how it works. There's a great monologue by the late scientist Richard Feynman from an interview called The Pleasure of Finding Things Out that says it much better than I ever could. *FWIW, I dislike the word theory, it's a dead thing that belongs in dusty libraries. Harmony - the living, breathing, application of the rules of music - is much more preferable.

Devil's advocate suggestion... unless they're moving a song down from E to Eb and you really want the lowest possible octave* then just save your money and use your brain. Transposing a song down a semitone shouldn't be that much of a headache once you've tried it a couple of times. *assuming you're playing a 4-string bass in standard tuning, that is.

Even though I transcribe a lot of music I always feel like my ears aren't as good as I want them to be. These are the best ways that I've found to get them in shape without spending any money:

Exactly. A lot of people parrot the very tired 'music is a language' analogy but then don't make any real effort to actually learn the language in a meaningful way. No point learning fancy words if you don't know what they mean or understand when to use them.

This might not seem like the answer that you want, but... A lot of people, bass players in particular, struggle to understand harmony because they don't have a solid understanding of intervals (how two notes relate to each other) and basic triads first. It's going to be hard to get your head around 7th chord harmony and the devices that Marcus Miller is talking about if you skip over the much less exciting foundation work. A good starting point is to look up the intervals of the major scale (major 2nd, major 3rd, perfect 4th, perfect 5th, major 6th, major 7th) and understand how those intervals are constructed (the musical spelling and grammar, if you like), how they're played on the bass, and how each one sounds. You don't necessarily need a resource that's aimed at bassists, and it can be helpful to work on some of this stuff at a piano/keyboard as it can provide an easier way to visualise certain things compared to the fretboard.

Those three strands of professional bassist all require slightly different skill sets (FWIW most full time musicians that I know have to take lots of different sorts of work to survive). If you want to do theatre work/cruise ship stuff etc then being a decent reader is a must, some sessions require reading but many require fast ears and a broad knowledge of what to play in different styles. TAB does not exist in the professional world. Covers band work is probably the most easy to get into, but you need to memorise a lot of tunes (50-100ish to begin with) and be able to transpose them into different keys at short notice if you want to be able to dep with different bands. Being able to do backing vocals makes you way more employable. In general, professional players across the board tend to have a great sense of rhythm, decent ears, and have learned enough tunes over the years to be able to improvise a suitable bassline when asked to do so. You don't need tons of technical proficiency or slap bass pyrotechnics to get plenty of gigs and as long as your gear works well it doesn't matter how much it costs. It's true that a lot of work comes from your network, but it's not all down to being a people person; knowing the right people gets you the gig, knowing the right stuff means you keep getting called for that gig (and people think of you when they need a bassist for something else...).

If you've got two consecutive open strings descending then the left hand has to mute the higher one - in this case the D gets muted at the exact split second that you pluck the open A. This is a hard thing to get used to at first but it's hugely important. In general, the right hand takes care of all the strings below the one you're playing, the left hand mutes anything above the string being played.

Having dabbled in (and subsequently rejected) the E-C thing I got a lot out of working through parts of The Advancing Guitarist by Mick Goodrick; there's a couple of chapters that have some really heavy exercises on voice leading and some different ways of voicing chords (drop-2, etc) that might open up some new avenues for you - I found that working on spread triad voicings gave me a lot of useful material that could be applied even in a non-jazz situation.

The Functional Ear Trainer app is the best thing I've found for getting the hang of how intervals sound (and, more importantly, feel) within the context of a key, which is what you need if you're trying to work out songs on the fly. Having been playing for 20+ years this is the one thing I'd go back and do every day if I had the chance.