I owe like 30x my annual salary.For me I owe about 75 times my 2022 taxable income (2023 taxable income has not yet been determine,  but I expect it to not be significantly higher than 2022 - San Diego receive tax extension due to flooding). 

I am thinking the cash from the cash out refi is not taxable sense I have a new loan and I only owe taxes from the sale of $220k house minus - $170k new loan, 

Getting it in or out almost always results in a taxable event, and when an owner passes away, the beneficiaries would receive a step up in *stock value*, not a step up in the assets within the corporation.

Usually we have a few thousand dollars each year to invest after we max out these accounts (and it usually just goes into a taxable brokerage account where we use it to buy Berkshire Hathaway or Markel or some other stock that does not pay a dividend).That extra money was actually how we ended up buying this duplex - thought it might be a good way to diversify our investments.I was wondering if I could use net income from our duplex (assuming we have net income, and by net income I mean what we report to the IRS) to fund a solo 401k or some other kind of tax-advantaged plan?

As interest (taxable as regular income), capital gains (usually taxed at 15%), and basically return of principle (pry called something else but payment against her cost basis.) that’s tax free.

@Benjamin Aaker is on the money.Your income probably prevents you from using RE to offset your W2 income but you can still use deductions and depreciation to offset rental income.If you are looking to REI to reduce your taxable income on your W2 earnings REI won’t help much at all.One needs to be precise when discussing tax law.

According to @Jay Hinrichs, the back yard might be a separate S corporation as developer for ordinary taxable gains and the front part a 1031 exchange for capital gains. 

We can show you how to shift some of your investments assets from the taxable to the non taxable bucket.  

Or would that result in a taxable event because my mortgage would go from 150k to 0? 

By accelerating your depreciation schedules, you reduce your taxable income which in turn increases your operating cash flow.With the increased popularity, it has created an increase in cost segregation companies and technologies all trying to offer quick and inexpensive reports.