Almost nothing emulates an LS3/5a. I bought my first pair in 1976 and used them as main speakers more often then not until 2002. I have four suggestions: 1. Buy a used pair of LS3/5as; 2/ Spendor S3/5 or S3/5se, depending on your preference; Zu Tone; Silverline SR16.
The Spendor is as close as you can get to an LS3/5a without buying an LS3/5a, yet still it is decidely less "vintage." The Zu Tone shares the BBC monitor's pinpoint imaging and ability to disappear while offering more integrated transient consistency than the LS3/5a and 101/db/w/m efficiency to be useful with tonally superior SET amps. The Silverline SR16 is as close as you can get to an LS3/5a with greater powerhandling and some bass extension.
Phil