Update? What's the progress so far?
I hope you didn't cancel the request. IF your income is correct AMEX will most likely counter or approve.
To answer your question, they were denying the 45k CLI. The 50K they felt was excessive and to continue they want 4506. This doesn't mean the preset amount for 4506 is 50K. It simply means at this time for YOU and YOUR account only AMEX feels 50k is excessive without additional documentation.
As far as the 4506 request, there's a lot of confusion. There's no preset amount that keeps the income verification request (IVR) prompts away.
There's four main type of ways to receive a 4506T IVR.
1) Simply requesting a CLI might prompt IVR, doesn't matter how much for some accounts. Could be 5k or 50k, some accounts just get flagged. The 25K - 35K CLI are myths that are refutable by the many accounts that were flagged under 10k.
2) Spending, yeah it looks great you're spending and paying it back. Big spenders are ideal for IVR because those are the ones that put the bank at risk. What I mean by "Big Spender", you're holding/spending 40% or more on any given AMEX card. Some of you use your cards for business purchases, I'm going to let you in on a secret NO BANK likes that. Some banks even prohibit it in their Terms. Using cards that's not business cards to fund your business is identical to investing with credit. Credit card issuers even made the government make brokers stop accepting credit as a means to fund brokerage accounts because that's investing with credit. The business cards is still investing with credit but those are allowed because that's what those are made for.
3) Unusual spending habits. You can't go from normally spending 3k a month to now 14k. Sensitive Banks like AMEX view this financial change and perceive you're relying on credit more then income. You don't want any lender thinking this! Your monthly debt should never exceed a certain percentage of your MONTHLY income. Any amount above this ghost percentage increases the risk of default.
4) This fourth reason is the reason that makes account holders angry with AMEX. Flagged for no reason! Yup, all those that received CLI and was never flagged doesn't mean your in the clear. AMEX has a well known - established history for flagging accounts for what seems to be no reason at all.
I'm going to say one more I found out the hard way. No matter what your credit utilization (CU) is AMEX is the only bank I know that will frown from balances across multiple cards. I had a CU below 5 percent with balances spread across all cards and lets just say AMEX wasn't my friend that month until a rep told me when my report was pulled the reason why.
Hey I get it, we're all trying to beef up our credit profiles (CP) with high limit cards. If you don't mind sending in Tax Transcripts AMEX is actually a good choice. For others that nudged their income up a little or simply not interested in sending personal Tax Info AMEX is probably not the card you want to have as the dominate HIgh Limit card in your CP. IF/When AMEX IVR your account and you choose not to send in the Taxes.AMEX or You will close that account and that's going to be a major blow to your CP.
Just have faith and things will work out with time. 2 holiday weekends most likely led to the longer than usual processing of the request. 
