I'm not quite sure I understand the amorization detail you are pointing out, but I'll try to answer anyway.
I believe this is a personal income tax question, not a business tax question. Let's call the business Startup X. There are two possible scenarios I know of:
1) Startup X is organized as an S-Corp. The owners will each receive a K-1. the total profit (or loss) for the year is split as a percentage of ownership (not a percentage of raw dollars invested, unless of course - coincidentally - all investment occurred with the same valuation).
On the K-1 will be the losses for that year. If there were assets that were being depreciated over a series of years, the disposal of the assets will cause the rest of the depreciation to occur in this year. Example: you bought a $12k machine and depreciated it over 3 years. The business fails in year 2. The first $4k already showed up on the K-1 of the owners from the first year. The machine is sold for $2k. Net, there is $6k worth of loss that shows up on the K-1 from this machine.
Each individual owner will place the K-1 loss on their Schedule E of their income return. The total loss they are allowed to take against their regular income in the same year will be determined by other factors (mostly how much income they had), but you can roll over the difference to the next year.
All K-1s (from the current year and past years) will have an adjustment to the investment basis. In addition to reporting the business income loss from shutting the company down on your Schedule E, you also get to report the investment loss, but minus any basis adjustments.
2) The second scenario is that Startup X was not organized as a S-corp (S corp is a Federal designation - but corporations are organized at the state level; a little confusing, but if you're not an S, then for Federal purposes, you're this type).
This scenario is much simpler for the owners: the business losses are irrelevant to the owners income tax filing, since it's not a pass-through entity. You just take the entire investment you made (amount of equity that resulted in is irrelevant) and write it off as an investment loss. Imagine buying IBM shares and then disposing of them for $0 - it's basically that scenario on your taxes.
Hope this helps more than it confuses. Startup X itself will not own any income taxes, but that does not mean it will not owe any taxes unfortunately. Many states have filing fees at a minimum. Make sure you file all taxes/fees in your state correctly. It once took me 3 years to shut down a subsidiary. By the time the state got back to me explaining how I was not 100% done, I had to keep filing the next year's tax filings. Until the company is shut down, it exists - and you are responsible for all filing requirements for every year it exists (regardless of whether the bank account is $0). It's July, so you should have enough time - but be diligent.
Notice that in the second scenario (which I'm guessing you are in), all the business losses you've been accumulating get basically wiped out when you shut it down. In some cases, these accumulated losses are an asset of sorts - a profitable company could use these losses to offset their income and save on their business income taxes. This is not as simple as it sounds - there are a lot of restrictions on being able to do this. Accountants will delight in charging you to calculate a "test" to see if this is possible. If the investment was $100k and no one has suggested this to you before now, I don't know if I'd suggest looking into it - you definitely need the acquiring company to pay for all the accounting fees. Just be willing to be tax inefficient and walk away.
However, I will say that it's sometimes easier to sell a company for $1 than deal with shutting it down. It clears you of all subsequent tax liability (the new owners get that) with one piece of paper. So if there's any value to what you have on paper that someone else can use, it's potentially worth basically giving it away, if for no other reason than to avoid the headaches of an improper shutdown.