don't know where you got your "last legs" for SDT. only 2 years old and 18 more to go. i have both SDT and CHKR. both are mostly doing minimum distributions, and the subordinated units held by the issuers are getting killed relative to unsubordinated publicly held units.
both have been beaten down recently and are targets for accumulation.
unless you enjoy massive paperwork that is clear as concrete(K-1 trust documentation), keep any or all in a tax exempt account like an IRA.
these things will get a kick in the pants when oil and gas go back up.