Most hosts offering those unmeters plans have a fineprint saying that you're not supposed to use them just for offering files for download or that your server is able to burst at the highest speed for short period of time but if the usage is high you're supposed to upgrade to some bigger plans.
If you buy a dedicated server, usually providers specify the exact limit of bandwidth you can use or they say it's unmetered but shared between several other servers and you can expect to be able to use that many GB of bandwidth.
100tb.com does really offer those servers with 100TB of bandwidth and there is no "fineprint" - some people also posted graphs showing their servers going up to 800 mbps for relatively long periods of time.
If you check the offers section here, you may also be able to find servers cheaper than those offered by 100tb.com but with 100mbps unmetered connections (which gives you a maximum of 33 TB a month) though if your site already uses 15 TB of bandwidth, you may have peaks of download over 100mbps so your site may become sluggish.
You basically have two options : go with a server on gigabit port and lots of bandwidth or go with several (possibly) cheaper servers. It would make sense to go with several cheaper ones only if the number of simultaneous downloads is so high, the hard drives wouldn't be able to seek and get the data as fast as the connection - but at this point I'm sure you could also contact 100tb's sales and ask them to add several hard drives to your server and solve the issue.