According to Gildersleeve and Lodge, Latin grammar §296, Remark 1.b.2:
The Abl. [of comparison] is very common in negative sentences, and is used exclusively in negative relative sentences.
So, although a noun or pronoun other than a relative pronoun is often used as an ablative of comparison in negative sentences, the relative pronoun is used in this way only in negative sentences.
So according to the second clause of this rule, if the sentence is both negative and relative, only the ablative is used, never quam; and according to the first clause, if the sentence is negative but non-relative, either quam or ablative can be used (and the ablative is 'very common'). Of course, this rule doesn't address the remaining two permutations: a sentence that is both non-negative and non-relative, and a sentence that is non-negative but relative; in both these cases, the implication is that either quam or ablative can also be used.
Here are two examples of a non-negative relative sentence that uses the ablative:
Cicero, Pro Milone 33.90:
(The relative here is a so-called 'connecting relative.')
an ille praetor, ille vero consul, si modo haec templa atque ipsa moenia stare eo vivo tam diu et consulatum eius exspectare potuissent, ille denique vivus mali nihil fecisset cui mortuo unus ex suis satellitibus curiam incenderit? quo quid miserius, quid
acerbius, quid luctuosius vidimus?
Would that man when praetor, much more when consul, provided only that these temples and these walls could have stood so long if he had been alive, and could have remained till his consulship; would he, I say, if alive, have done no harm, when even after he was dead he burned the senate-house, one of his satellites, Sextus Clodius, being the ringleader in the tumult? What more miserable, more grievous, more bitter sight have we ever seen than that?
Cicero, Philippicae 1.4.10:
exque eo primum cognovi quae Kalendis Sextilibus in senatu fuisset L. Pisonis oratio: qui quamquam parum erat – id enim ipsum a Bruto audieram – a quibus debuerat adiutus, tamen et Bruti testimonio – quo quid potest esse gravius? – et omnium praedicatione quos postea vidi magnam mihi videbatur gloriam consecutus.
And it was from him that I first heard what had been the language of Lucius Piso, in the senate of August; who, although he was but little assisted (for that I heard from Brutus himself) by those who ought to have seconded him, still according to the testimony of Brutus, (and what evidence can be more trustworthy?) and to the avowal of every one whom I saw afterwards, appeared to me to have gained great credit.
(Translations by C. D. Yonge; from the Perseus website)